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Aircraft on display
No operational B-2s have been retired by the Air Force to be put on display. B-2s have made occasional appearances on ground display at various air shows.
B-2 test article (s/n AT-1000), the second of two built without engines or instruments and used for static testing, was placed on display in 2004 at the National Museum of the United States Air Force near Dayton, Ohio. The test article passed all structural testing requirements before the airframe failed. The museum's restoration team spent over a year reassembling the fractured airframe. The display airframe is marked to resemble Spirit of Ohio (S/N 82-1070), the B-2 used to test the design's ability to withstand extreme heat and cold. The exhibit features Spirit of Ohios nose wheel door, with its Fire and Ice artwork, which was painted and signed by the technicians who performed the temperature testing. The restored test aircraft is on display in the museum's "Cold War Gallery".
Specifications (B-2A Block 30)
Individual aircraft
Notable appearances in media | Northrop B-2 Spirit | Wikipedia | 224 | 4396 | https://en.wikipedia.org/wiki/Northrop%20B-2%20Spirit | Technology | Specific aircraft | null |
Beavers (genus Castor) are large, semiaquatic rodents of the Northern Hemisphere. There are two existing species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). Beavers are the second-largest living rodents, after capybaras, weighing up to . They have stout bodies with large heads, long chisel-like incisors, brown or gray fur, hand-like front feet, webbed back feet, and tails that are flat and scaly. The two species differ in skull and tail shape and fur color. Beavers can be found in a number of freshwater habitats, such as rivers, streams, lakes and ponds. They are herbivorous, consuming tree bark, aquatic plants, grasses and sedges.
Beavers build dams and lodges using tree branches, vegetation, rocks and mud; they chew down trees for building material. Dams restrict water flow, and lodges serve as shelters. Their infrastructure creates wetlands used by many other species, and because of their effect on other organisms in the ecosystem, beavers are considered a keystone species. Adult males and females live in monogamous pairs with their offspring. After their first year, the young help their parents repair dams and lodges; older siblings may also help raise newly born offspring. Beavers hold territories and mark them using scent mounds made of mud, debris, and castoreum—a liquid substance excreted through the beaver's urethra-based castor sacs. Beavers can also recognize their kin by their anal gland secretions and are more likely to tolerate them as neighbors.
Historically, beavers have been hunted for their fur, meat, and castoreum. Castoreum has been used in medicine, perfume, and food flavoring; beaver pelts have been a major driver of the fur trade. Before protections began in the 19th and early 20th centuries, overhunting had nearly exterminated both species. Their populations have since rebounded, and they are listed as species of least concern by the IUCN Red List of mammals. In human culture, the beaver symbolizes industriousness, especially in connection with construction; it is the national animal of Canada. | Beaver | Wikipedia | 452 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Etymology
The English word beaver comes from the Old English word or and is connected to the German word and the Dutch word . The ultimate origin of the word is an Indo-European root for . Cognates of beaver is the source for several European placenames, including those of Beverley, Bièvres, Biberbach, Biebrich, Bibra, Bibern, Bibrka, Bobr, Bober, Bóbrka, Bjurholm, Bjurälven, and Bjurum. The genus name Castor has its origin in the Greek word and translates as .
Taxonomy
Carl Linnaeus coined the genus name Castor in 1758 as well as the specific (species) epithet fiber for the Eurasian species. German zoologist Heinrich Kuhl coined C. canadensis in 1820, many scientists considered both names synonymous for one same species until the 1970s, when chromosomal evidence became available confirming both as separate where the Eurasian has 48 chromosomes, while the North American has 40.) The difference in chromosome numbers prevents them from interbreeding. Twenty-five subspecies have been classified for C. canadensis, and nine have been classified for C. fiber.
There are two extant species: the North American beaver (Castor canadensis) and the Eurasian beaver (C. fiber). The Eurasian beaver is slightly longer and has a more lengthened skull, triangular nasal cavities (as opposed to the square ones of the North American species), a lighter fur color, and a narrower tail.
Evolution
Beavers belong to the rodent suborder Castorimorpha, along with Heteromyidae (kangaroo rats and kangaroo mice), and the gophers. Modern beavers are the only extant members of the family Castoridae. They originated in North America in the late Eocene and colonized Eurasia via the Bering Land Bridge in the early Oligocene, coinciding with the Grande Coupure, a time of significant changes in animal species around 33million years ago (myr). | Beaver | Wikipedia | 417 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
The more basal castorids had several unique features: more complex occlusion between cheek teeth, parallel rows of upper teeth, premolars that were only slightly smaller than molars, the presence of a third set of premolars (P3), a hole in the stapes of the inner ear, a smooth palatine bone (with the palatine opening closer to the rear end of the bone), and a longer snout. More derived castorids have less complex occlusion, upper tooth rows that create a V-shape towards the back, larger second premolars compared to molars, absence of a third premolar set and stapes hole, a more grooved palatine (with the opening shifted towards the front), and reduced incisive foramen. Members of the subfamily Palaeocastorinae appeared in late-Oligocene North America. This group consisted primarily of smaller animals with relatively large front legs, a flattened skull, and a reduced tail—all features of a fossorial (burrowing) lifestyle.
In the early Miocene (about 24 mya), castorids evolved a semiaquatic lifestyle. Members of the subfamily Castoroidinae are considered to be a sister group to modern beavers, and included giants like Castoroides of North America and Trogontherium of Eurasia. Castoroides is estimated to have had a length of and a weight of . Fossils of one genus in Castoroidinae, Dipoides, have been found near piles of chewed wood, though Dipoides appears to have been an inferior woodcutter compared to Castor. Researchers suggest that modern beavers and Castoroidinae shared a bark-eating common ancestor. Dam and lodge-building likely developed from bark-eating, and allowed beavers to survive in the harsh winters of the subarctic. There is no conclusive evidence for this behavior occurring in non-Castor species. | Beaver | Wikipedia | 394 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
The genus Castor likely originated in Eurasia. The earliest fossil remains appear to be C. neglectus, found in Germany and dated 12–10 mya. Mitochondrial DNA studies place the common ancestor of the two living species at around 8 mya. The ancestors of the North American beaver would have crossed the Bering Land Bridge around 7.5 mya. Castor may have competed with members of Castoroidinae, which led to niche differentiation. The fossil species C. praefiber was likely an ancestor of the Eurasian beaver. C. californicus from the Early Pleistocene of North America was similar to but larger than the extant North American beaver.'
Characteristics
Beavers are the second-largest living rodents, after capybaras. They have a head–body length of , with a tail, a shoulder height of , and generally weigh , but can be as heavy as . Males and females are almost identical externally. Their bodies are streamlined like marine mammals and their robust build allows them to pull heavy loads. A beaver coat has 12,000–23,000 hairs/cm2 (77,000–148,000 hairs/in2) and functions to keep the animal warm, to help it float in water, and to protect it against predators. Guard hairs are long and typically reddish brown, but can range from yellowish brown to nearly black. The underfur is long and dark gray. Beavers molt every summer. | Beaver | Wikipedia | 293 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers have large skulls with powerful chewing muscles. They have four chisel-shaped incisors that continue to grow throughout their lives. The incisors are covered in a thick enamel that is colored orange or reddish-brown by iron compounds. The lower incisors have roots that are almost as long as the entire lower jaw. Beavers have one premolar and three molars on all four sides of the jaws, adding up to 20 teeth. The molars have meandering ridges for grinding woody material. The eyes, ears and nostrils are arranged so that they can remain above water while the rest of the body is submerged. The nostrils and ears have valves that close underwater, while nictitating membranes cover the eyes. To protect the larynx and trachea from water flow, the epiglottis is contained within the nasal cavity instead of the throat. In addition, the back of the tongue can rise and create a waterproof seal. A beaver's lips can close behind the incisors, preventing water from entering their mouths as they cut and bite onto things while submerged.
The beaver's front feet are dexterous, allowing them to grasp and manipulate objects and food, as well as dig. The hind feet are larger and have webbing between the toes, and the second innermost toe has a "double nail" used for grooming. Beavers can swim at ; only their webbed hind feet are used to swim, while the front feet fold under the chest. On the surface, the hind limbs thrust one after the other; while underwater, they move at the same time. Beavers are awkward on land but can move quickly when they feel threatened. They can carry objects while walking on their hind legs.
The beaver's distinctive tail has a conical, muscular, hairy base; the remaining two-thirds of the appendage is flat and scaly. The tail has multiple functions: it provides support for the animal when it is upright (such as when chewing down a tree), acts as a rudder when it is swimming, and stores fat for winter. It also has a countercurrent blood vessel system which allows the animal to lose heat in warm temperatures and retain heat in cold temperatures. | Beaver | Wikipedia | 455 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
The beaver's sex organs are inside the body, and the male's penis has a cartilaginous baculum. They have only one opening, a cloaca, which is used for reproduction, scent-marking, defecation, and urination. The cloaca evolved secondarily, as most mammals have lost this feature, and may reduce the area vulnerable to infection in dirty water. The beaver's intestine is six times longer than its body, and the caecum is double the volume of its stomach. Microorganisms in the caecum allow them to process around 30 percent of the cellulose they eat. A beaver defecates in the water, leaving behind balls of sawdust. Female beavers have four mammary glands; these produce milk with 19 percent fat, a higher fat content than other rodents. Beavers have two pairs of glands: castor sacs, which are part of the urethra, and anal glands. The castor sacs secrete castoreum, a liquid substance used mainly for marking territory. Anal glands produce an oily substance which the beaver uses as a waterproof ointment for its coat. The substance plays a role in individual and family recognition. Anal secretions are darker in females than males among Eurasian beavers, while the reverse is true for the North American species.
Compared to many other rodents, a beaver's brain has a hypothalamus that is much smaller than the cerebrum; this indicates a relatively advanced brain with higher intelligence. The cerebellum is large, allowing the animal to move within a three-dimensional space (such as underwater) similar to tree-climbing squirrels. The neocortex is devoted mainly to touch and hearing. Touch is more advanced in the lips and hands than the whiskers and tail. Vision in the beaver is relatively poor; the beaver eye cannot see as well underwater as an otter. Beavers have a good sense of smell, which they use for detecting land predators and for inspecting scent marks, food, and other individuals. | Beaver | Wikipedia | 431 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers can hold their breath for as long as 15 minutes but typically remain underwater for no more than five or six minutes. Dives typically last less than 30 seconds and are usually no more than deep. When diving, their heart rate decreases to 60 beats per minute, half its normal pace, and blood flow is directed more towards the brain. A beaver's body also has a high tolerance for carbon dioxide. When surfacing, the animal can replace 75 percent of the air in its lungs in one breath, compared to 15 percent for a human.
Distribution and status
The IUCN Red List of mammals lists both beaver species as least concern. The North American beaver is widespread throughout most of the United States and Canada and can be found in northern Mexico. The species was introduced to Finland in 1937 (and then spread to northwestern Russia) and to Tierra del Fuego, Patagonia, in 1946. , the introduced population of North American beavers in Finland has been moving closer to the habitat of the Eurasian beaver. Historically, the North American beaver was trapped and nearly extirpated because its fur was highly sought after. Protections have allowed the beaver population on the continent to rebound to an estimated 6–12million by the late 20th century; still far lower than the originally estimated 60–400million North American beavers before the fur trade. The introduced population in Tierra del Fuego is estimated at 35,000–50,000 individuals .
The Eurasian beaver's range historically included much of Eurasia, but was decimated by hunting by the early 20th century. In Europe, beavers were reduced to fragmented populations, with combined population numbers being estimated at 1,200 individuals for the Rhône of France, the Elbe in Germany, southern Norway, the Neman river and Dnieper Basin in Belarus, and the Voronezh river in Russia. The beaver has since recolonized parts of its former range, aided by conservation policies and reintroductions. Beaver populations now range across western, central, and eastern Europe, and western Russia and the Scandinavian Peninsula. Beginning in 2009, beavers have been successfully reintroduced to parts of Great Britain. , the total Eurasian beaver population in Europe was estimated at over one million. Small native populations are also present in Mongolia and northwestern China; their numbers were estimated at 150 and 700, respectively, . Under New Zealand's Hazardous Substances and New Organisms Act 1996, beavers are classed as a "prohibited new organism" preventing them from being introduced into the country.
Ecology | Beaver | Wikipedia | 510 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers live in freshwater ecosystems such as rivers, streams, lakes and ponds. Water is the most important component of beaver habitat; they swim and dive in it, and it provides them refuge from land predators. It also restricts access to their homes and allows them to move building objects more easily. Beavers prefer slower moving streams, typically with a gradient (steepness) of one percent, though they have been recorded using streams with gradients as high as 15 percent. Beavers are found in wider streams more often than in narrower ones. They also prefer areas with no regular flooding and may abandon a location for years after a significant flood.
Beavers typically select flat landscapes with diverse vegetation close to the water. North American beavers prefer trees being or less from the water, but will roam several hundred meters to find more. Beavers have also been recorded in mountainous areas. Dispersing beavers will use certain habitats temporarily before finding their ideal home. These include small streams, temporary swamps, ditches, and backyards. These sites lack important resources, so the animals do not stay there permanently. Beavers have increasingly settled at or near human-made environments, including agricultural areas, suburbs, golf courses, and shopping malls.
Beavers have an herbivorous and a generalist diet. During the spring and summer, they mainly feed on herbaceous plant material such as leaves, roots, herbs, ferns, grasses, sedges, water lilies, water shields, rushes, and cattails. During the fall and winter, they eat more bark and cambium of woody plants; tree and shrub species consumed include aspen, birch, oak, dogwood, willow and alder. There is some disagreement about why beavers select specific woody plants; some research has shown that beavers more frequently select species which are more easily digested, while others suggest beavers principally forage based on stem size. Beavers may cache their food for the winter, piling wood in the deepest part of their pond where it cannot be reached by other browsers. This cache is known as a "raft"; when the top becomes frozen, it creates a "cap". The beaver accesses the raft by swimming under the ice. Many populations of Eurasian beaver do not make rafts, but forage on land during winter. | Beaver | Wikipedia | 470 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers usually live up to 10 years. Felids, canids, and bears may prey upon them. Beavers are protected from predators when in their lodges, and prefer to stay near water. Parasites of the beaver include the bacteria Francisella tularensis, which causes tularemia; the protozoan Giardia duodenalis, which causes giardiasis (beaver fever); and the beaver beetle and mites of the genus Schizocarpus. They have also been recorded to be infected with the rabies virus.
Infrastructure
Beavers need trees and shrubs to use as building material for dams, which restrict flowing water to create a pond for them to live in, and for lodges, which act as shelters and refuges from predators and the elements. Without such material, beavers dig burrows into a bank to live. Dam construction begins in late summer or early fall, and they repair them whenever needed. Beavers can cut down trees up to wide in less than 50 minutes. Thicker trees, at wide or more, may not fall for hours. When chewing down a tree, beavers switch between biting with the left and right side of the mouth. Tree branches are then cut and carried to their destination with the powerful jaw and neck muscles. Other building materials, like mud and rocks, are held by the forelimbs and tucked between the chin and chest.
Beavers start building dams when they hear running water, and the sound of a leak in a dam triggers them to repair it. To build a dam, beavers stack up relatively long and thick logs between banks and in opposite directions. Heavy rocks keep them stable, and grass is packed between them. Beavers continue to pile on more material until the dam slopes in a direction facing upstream. Dams can range in height from to and can stretch from to several hundred meters long. Beaver dams are more effective in trapping and slowly leaking water than man-made concrete dams. Lake-dwelling beavers do not need to build dams. | Beaver | Wikipedia | 406 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers make two types of lodges: bank lodges and open-water lodges. Bank lodges are burrows dug along the shore and covered in sticks. The more complex freestanding, open-water lodges are built over a platform of piled-up sticks. The lodge is mostly sealed with mud, except for a hole at the top which acts as an air vent. Both types are accessed by underwater entrances. The above-water space inside the lodge is known as the "living chamber", and a "dining area" may exist close to the water entrance. Families routinely clean out old plant material and bring in new material.
North American beavers build more open-water lodges than Eurasian beavers. Beaver lodges built by new settlers are typically small and sloppy. More experienced families can build structures with a height of and an above-water diameter of . A lodge sturdy enough to withstand the coming winter can be finished in just two nights. Both lodge types can be present at a beaver site. During the summer, beavers tend to use bank lodges to keep cool. They use open-water lodges during the winter. The air vent provides ventilation, and newly added carbon dioxide can be cleared in an hour. The lodge remains consistent in oxygen and carbon dioxide levels from season to season.
Beavers in some areas will dig canals connected to their ponds. The canals fill with groundwater and give beavers access and easier transport of resources, as well as allow them to escape predators. These canals can stretch up to wide, deep, and over long. It has been hypothesized that beavers' canals are not only transportation routes but an extension of their "central place" around the lodge and/or food cache. As they drag wood across the land, beavers leave behind trails or "slides", which they reuse when moving new material.
Environmental effects | Beaver | Wikipedia | 372 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
The beaver works as an ecosystem engineer and keystone species, as its activities can have a great impact on the landscape and biodiversity of an area. Aside from humans, few other extant animals appear to do more to shape their environment. When building dams, beavers alter the paths of streams and rivers, allowing for the creation of extensive wetland habitats. In one study, beavers were associated with large increases in open-water areas. When beavers returned to an area, 160% more open water was available during droughts than in previous years, when they were absent. Beaver dams also lead to higher water tables in mineral soil environments and in wetlands such as peatlands. In peatlands particularly, their dams stabilize the constantly changing water levels, leading to greater carbon storage.
Beaver ponds, and the wetlands that succeed them, remove sediments and pollutants from waterways, and can stop the loss of important soils. These ponds can increase the productivity of freshwater ecosystems by accumulating nitrogen in sediments. Beaver activity can affect the temperature of the water; in northern latitudes, ice thaws earlier in the warmer beaver-dammed waters. Beavers may contribute to climate change. In Arctic areas, the floods they create can cause permafrost to thaw, releasing methane into the atmosphere.
As wetlands are formed and riparian habitats are enlarged, aquatic plants colonize the newly available watery habitat. One study in the Adirondacks found that beaver engineering lead to an increase of more than 33 percent in herbaceous plant diversity along the water's edge. Another study in semiarid eastern Oregon found that the width of riparian vegetation on stream banks increased several-fold as beaver dams watered previously dry terraces adjacent to the stream. Riparian ecosystems in arid areas appear to sustain more plant life when beaver dams are present. Beaver ponds act as a refuge for riverbank plants during wildfires, and provide them with enough moisture to resist such fires. Introduced beavers at Tierra del Fuego have been responsible for destroying the indigenous forest. Unlike trees in North America, many trees in South America cannot grow back after being cut down. | Beaver | Wikipedia | 427 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beaver activity impacts communities of aquatic invertebrates. Damming typically leads to an increase of slow or motionless water species, like dragonflies, oligochaetes, snails, and mussels. This is to the detriment of rapid water species like black flies, stoneflies, and net-spinning caddisflies. Beaver floodings create more dead trees, providing more habitat for terrestrial invertebrates like Drosophila flies and bark beetles, which live and breed in dead wood. The presence of beavers can increase wild salmon and trout populations, and the average size of these fishes. These species use beaver habitats for spawning, overwintering, feeding, and as havens from changes in water flow. The positive effects of beaver dams on fish appear to outweigh the negative effects, such as blocking of migration. Beaver ponds have been shown to be beneficial to frog populations by protecting areas for larvae to mature in warm water. The stable waters of beaver ponds also provide ideal habitat for freshwater turtles.
Beavers help waterfowl by creating increased areas of water. The widening of the riparian zone associated with beaver dams has been shown to increase the abundance and diversity of birds favoring the water's edge, an impact that may be especially important in semi-arid climates. Fish-eating birds use beaver ponds for foraging, and in some areas, certain species appear more frequently at sites where beavers were active than at sites with no beaver activity. In a study of Wyoming streams and rivers, watercourses with beavers had 75 times as many ducks as those without. As trees are drowned by rising beaver impoundments, they become an ideal habitat for woodpeckers, which carve cavities that may be later used by other bird species. Beaver-caused ice thawing in northern latitudes allows Canada geese to nest earlier.
Other semi-aquatic mammals, such as water voles, muskrats, minks, and otters, will shelter in beaver lodges. Beaver modifications to streams in Poland create habitats favorable to bat species that forage at the water surface and "prefer moderate vegetation clutter". Large herbivores, such as some deer species, benefit from beaver activity as they can access vegetation from fallen trees and ponds.
Behavior | Beaver | Wikipedia | 457 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers are mainly nocturnal and crepuscular, and spend the daytime in their shelters. In northern latitudes, beaver activity is decoupled from the 24-hour cycle during the winter, and may last as long as 29 hours. They do not hibernate during winter, and spend much of their time in their lodges.
Family life
The core of beaver social organization is the family, which is composed of an adult male and an adult female in a monogamous pair and their offspring. Beaver families can have as many as ten members; groups about this size require multiple lodges. Mutual grooming and play fighting maintain bonds between family members, and aggression between them is uncommon.
Adult beavers mate with their partners, though partner replacement appears to be common. A beaver that loses its partner will wait for another one to come by. Estrus cycles begin in late December and peak in mid-January. Females may have two to four estrus cycles per season, each lasting 12–24 hours. The pair typically mate in the water and to a lesser extent in the lodge, for half a minute to three minutes.
Up to four young, or kits, are born in spring and summer, after a three or four-month gestation. Newborn beavers are precocial with a full fur coat, and can open their eyes within days of birth. Their mother is the primary caretaker, while their father maintains the territory. Older siblings from a previous litter also play a role.
After they are born, the kits spend their first one to two months in the lodge. Kits suckle for as long as three months, but can eat solid food within their second week and rely on their parents and older siblings to bring it to them. Eventually, beaver kits explore outside the lodge and forage on their own, but may follow an older relative and hold onto their backs. After their first year, young beavers help their families with construction. Beavers sexually mature around 1.5–3 years. They become independent at two years old, but remain with their parents for an extra year or more during times of food shortage, high population density, or drought.
Territories and spacing | Beaver | Wikipedia | 439 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers typically disperse from their parental colonies during the spring or when the winter snow melts. They often travel less than , but long-distance dispersals are not uncommon when previous colonizers have already exploited local resources. Beavers are able to travel greater distances when free-flowing water is available. Individuals may meet their mates during the dispersal stage, and the pair travel together. It may take them weeks or months to reach their final destination; longer distances may require several years. Beavers establish and defend territories along the banks of their ponds, which may be in length.
Beavers mark their territories by constructing scent mounds made of mud and vegetation, scented with castoreum. Those with many territorial neighbors create more scent mounds. Scent marking increases in spring, during the dispersal of yearlings, to deter interlopers. Beavers are generally intolerant of intruders and fights may result in deep bites to the sides, rump, and tail. They exhibit a behavior known as the "dear enemy effect"; a territory-holder will investigate and become familiar with the scents of its neighbors and react more aggressively to the scents of strangers passing by. Beavers are also more tolerant of individuals that are their kin. They recognize them by using their keen sense of smell to detect differences in the composition of anal gland secretions. Anal gland secretion profiles are more similar among relatives than unrelated individuals.
Communication
Beavers within a family greet each other with whines. Kits will attract the attention of adults with mews, squeaks, and cries. Defensive beavers produce a hissing growl and gnash their teeth. Tail slaps, which involve an animal hitting the water surface with its tail, serve as alarm signals warning other beavers of a potential threat. An adult's tail slap is more successful in alerting others, who will escape into the lodge or deeper water. Juveniles have not yet learned the proper use of a tail slap, and hence are normally ignored. Eurasian beavers have been recorded using a territorial "stick display", which involves individuals holding up a stick and bouncing in shallow water.
Interactions with humans | Beaver | Wikipedia | 429 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers sometimes come into conflict with humans over land use; individual beavers may be labeled as "nuisance beavers". Beavers can damage crops, timber stocks, roads, ditches, gardens, and pastures via gnawing, eating, digging, and flooding. They occasionally attack humans and domestic pets, particularly when infected with rabies, in defense of their territory, or when they feel threatened. Some of these attacks have been fatal, including at least one human death. Beavers can spread giardiasis ('beaver fever') by infecting surface waters, though outbreaks are more commonly caused by human activity.
Flow devices, like beaver pipes, are used to manage beaver flooding, while fencing and hardware cloth protect trees and shrubs from beaver damage. If necessary, hand tools, heavy equipment, or explosives are used to remove dams. Hunting, trapping, and relocation may be permitted as forms of population control and for removal of individuals. The governments of Argentina and Chile have authorized the trapping of invasive beavers in hopes of eliminating them. The ecological importance of beavers has led to cities like Seattle designing their parks and green spaces to accommodate the animals. The Martinez beavers became famous in the mid-2000s for their role in improving the ecosystem of Alhambra Creek in Martinez, California.
Zoos have displayed beavers since at least the 19th century, though not commonly. In captivity, beavers have been used for entertainment, fur harvesting, and for reintroduction into the wild. Captive beavers require access to water, substrate for digging, and artificial shelters. Archibald Stansfeld "Grey Owl" Belaney pioneered beaver conservation in the early 20th century. Belaney wrote several books, and was first to professionally film beavers in their environment. In 1931, he moved to a log cabin in Prince Albert National Park, where he was the "caretaker of park animals" and raised a beaver pair and their four offspring.
Commercial use | Beaver | Wikipedia | 395 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beavers have been hunted, trapped, and exploited for their fur, meat, and castoreum. Since the animals typically stayed in one place, trappers could easily find them and could kill entire families in a lodge. Many pre-modern people mistakenly thought that castoreum was produced by the testicles or that the castor sacs of the beaver were its testicles, and females were hermaphrodites. Aesop's Fables describes beavers chewing off their testicles to preserve themselves from hunters, which is impossible because a beaver's testicles are internal. This myth persisted for centuries, and was corrected by French physician Guillaume Rondelet in the 1500s. Beavers have historically been hunted and captured using deadfalls, snares, nets, bows and arrows, spears, clubs, firearms, and leg-hold traps. Castoreum was used to lure the animals.
Castoreum was used for a variety of medical purposes; Pliny the Elder promoted it as a treatment for stomach problems, flatulence, seizures, sciatica, vertigo, and epilepsy. He stated it could stop hiccups when mixed with vinegar, toothaches if mixed with oil (by administering into the ear opening on the same side as the tooth), and could be used as an antivenom. The substance has traditionally been prescribed to treat hysteria in women, which was believed to have been caused by a "toxic" womb. Castoreum's properties have been credited to the accumulation of salicylic acid from willow and aspen trees in the beaver's diet, and has a physiological effect comparable to aspirin. Today, the medical use of castoreum has declined and is limited mainly to homeopathy. The substance is also used as an ingredient in perfumes and tinctures, and as a flavouring in food and drinks.
Various Native American groups have historically hunted beavers for food, they preferred its meat more than other red meats because of its higher calorie and fat content, and the animals remained plump in winter when they were most hunted. The bones were used to make tools. In medieval Europe, the Catholic Church considered the beaver to be part mammal and part fish, and allowed followers to eat the scaly, fishlike tail on meatless Fridays during Lent. Beaver tails were thus highly prized in Europe; they were described by French naturalist Pierre Belon as tasting like a "nicely dressed eel". | Beaver | Wikipedia | 504 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Beaver pelts were used to make hats; felters would remove the guard hairs. The number of pelts needed depended on the type of hat, with Cavalier and Puritan hats requiring more fur than top hats. In the late 16th century, Europeans began to deal in North American furs due to the lack of taxes or tariffs on the continent and the decline of fur-bearers at home. Beaver pelts caused or contributed to the Beaver Wars, King William's War, and the French and Indian War; the trade made John Jacob Astor and the owners of the North West Company very wealthy. For Europeans in North America, the fur trade was a driver of the exploration and westward exploration on the continent and contact with native peoples, who traded with them. The fur trade peaked between 1860 and 1870, when over 150,000 beaver pelts were purchased annually by the Hudson's Bay Company and fur companies in the United States. The contemporary global fur trade is not as profitable due to conservation, anti-fur and animal rights campaigns.
In culture
The beaver has been used to represent productivity, trade, tradition, masculinity, and respectability. | Beaver | Wikipedia | 233 | 4399 | https://en.wikipedia.org/wiki/Beaver | Biology and health sciences | Rodents | null |
Bears are carnivoran mammals of the family Ursidae (). They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout most of the Northern Hemisphere and partially in the Southern Hemisphere. Bears are found on the continents of North America, South America, and Eurasia. Common characteristics of modern bears include large bodies with stocky legs, long snouts, small rounded ears, shaggy hair, plantigrade paws with five nonretractile claws, and short tails.
While the polar bear is mostly carnivorous, and the giant panda is mostly herbivorous, the remaining six species are omnivorous with varying diets. With the exception of courting individuals and mothers with their young, bears are typically solitary animals. They may be diurnal or nocturnal and have an excellent sense of smell. Despite their heavy build and awkward gait, they are adept runners, climbers, and swimmers. Bears use shelters, such as caves and logs, as their dens; most species occupy their dens during the winter for a long period of hibernation, up to 100 days.
Bears have been hunted since prehistoric times for their meat and fur; they have also been used for bear-baiting and other forms of entertainment, such as being made to dance. With their powerful physical presence, they play a prominent role in the arts, mythology, and other cultural aspects of various human societies. In modern times, bears have come under pressure through encroachment on their habitats and illegal trade in bear parts, including the Asian bile bear market. The IUCN lists six bear species as vulnerable or endangered, and even least concern species, such as the brown bear, are at risk of extirpation in certain countries. The poaching and international trade of these most threatened populations are prohibited, but still ongoing. | Bear | Wikipedia | 393 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Etymology
The English word "bear" comes from Old English and belongs to a family of names for the bear in Germanic languages, such as Swedish , also used as a first name. This form is conventionally said to be related to a Proto-Indo-European word for "brown", so that "bear" would mean "the brown one". However, Ringe notes that while this etymology is semantically plausible, a word meaning "brown" of this form cannot be found in Proto-Indo-European. He suggests instead that "bear" is from the Proto-Indo-European word *ǵʰwḗr- ~ *ǵʰwér "wild animal". This terminology for the animal originated as a taboo avoidance term: proto-Germanic tribes replaced their original word for bear—arkto—with this euphemistic expression out of fear that speaking the animal's true name might cause it to appear. According to author Ralph Keyes, this is the oldest known euphemism.
Bear taxon names such as Arctoidea and Helarctos come from the ancient Greek ἄρκτος (arktos), meaning bear, as do the names "arctic" and "antarctic", via the name of the constellation Ursa Major, the "Great Bear", prominent in the northern sky.
Bear taxon names such as Ursidae and Ursus come from Latin Ursus/Ursa, he-bear/she-bear. The female first name "Ursula", originally derived from a Christian saint's name, means "little she-bear" (diminutive of Latin ursa). In Switzerland, the male first name "Urs" is especially popular, while the name of the canton and city of Bern is by legend derived from Bär, German for bear. The Germanic name Bernard (including Bernhardt and similar forms) means "bear-brave", "bear-hardy", or "bold bear". The Old English name Beowulf is a kenning, "bee-wolf", for bear, in turn meaning a brave warrior.
Taxonomy | Bear | Wikipedia | 427 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The family Ursidae is one of nine families in the suborder Caniformia, or "doglike" carnivorans, within the order Carnivora. Bears' closest living relatives are the pinnipeds, canids, and musteloids. (Some scholars formerly argued that bears are directly derived from canids and should not be classified as a separate family.) Modern bears comprise eight species in three subfamilies: Ailuropodinae (monotypic with the giant panda), Tremarctinae (monotypic with the spectacled bear), and Ursinae (containing six species divided into one to three genera, depending on the authority). Nuclear chromosome analysis show that the karyotype of the six ursine bears is nearly identical, each having 74 chromosomes (see Ursid hybrid), whereas the giant panda has 42 chromosomes and the spectacled bear 52. These smaller numbers can be explained by the fusing of some chromosomes, and the banding patterns on these match those of the ursine species, but differ from those of procyonids, which supports the inclusion of these two species in Ursidae rather than in Procyonidae, where they had been placed by some earlier authorities.
Evolution | Bear | Wikipedia | 259 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The earliest members of Ursidae belong to the extinct subfamily Amphicynodontinae, including Parictis (late Eocene to early middle Miocene, 38–18 Mya) and the slightly younger Allocyon (early Oligocene, 34–30 Mya), both from North America. These animals looked very different from today's bears, being small and raccoon-like in overall appearance, with diets perhaps more similar to that of a badger. Parictis does not appear in Eurasia and Africa until the Miocene. It is unclear whether late-Eocene ursids were also present in Eurasia, although faunal exchange across the Bering land bridge may have been possible during a major sea level low stand as early as the late Eocene (about 37 Mya) and continuing into the early Oligocene. European genera morphologically very similar to Allocyon, and to the much younger American Kolponomos (about 18 Mya), are known from the Oligocene, including Amphicticeps and Amphicynodon. There has been various morphological evidence linking amphicynodontines with pinnipeds, as both groups were semi-aquatic, otter-like mammals. In addition to the support of the pinniped–amphicynodontine clade, other morphological and some molecular evidence supports bears being the closest living relatives to pinnipeds. | Bear | Wikipedia | 291 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The raccoon-sized, dog-like Cephalogale is the oldest-known member of the subfamily Hemicyoninae, which first appeared during the middle Oligocene in Eurasia about 30 Mya. The subfamily includes the younger genera Phoberocyon (20–15 Mya), and Plithocyon (15–7 Mya). A Cephalogale-like species gave rise to the genus Ursavus during the early Oligocene (30–28 Mya); this genus proliferated into many species in Asia and is ancestral to all living bears. Species of Ursavus subsequently entered North America, together with Amphicynodon and Cephalogale, during the early Miocene (21–18 Mya). Members of the living lineages of bears diverged from Ursavus between 15 and 20 Mya, likely via the species Ursavus elmensis. Based on genetic and morphological data, the Ailuropodinae (pandas) were the first to diverge from other living bears about 19 Mya, although no fossils of this group have been found before about 11 Mya.
The New World short-faced bears (Tremarctinae) differentiated from Ursinae following a dispersal event into North America during the mid-Miocene (about 13 Mya). They invaded South America (≈2.5 or 1.2 Ma) following formation of the Isthmus of Panama. Their earliest fossil representative is Plionarctos in North America (c. 10–2 Ma). This genus is probably the direct ancestor to the North American short-faced bears (genus Arctodus), the South American short-faced bears (Arctotherium), and the spectacled bears, Tremarctos, represented by both an extinct North American species (T. floridanus), and the lone surviving representative of the Tremarctinae, the South American spectacled bear (T. ornatus). | Bear | Wikipedia | 409 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The subfamily Ursinae experienced a dramatic proliferation of taxa about 5.3–4.5 Mya, coincident with major environmental changes; the first members of the genus Ursus appeared around this time. The sloth bear is a modern survivor of one of the earliest lineages to diverge during this radiation event (5.3 Mya); it took on its peculiar morphology, related to its diet of termites and ants, no later than by the early Pleistocene. By 3–4 Mya, the species Ursus minimus appears in the fossil record of Europe; apart from its size, it was nearly identical to today's Asian black bear. It is likely ancestral to all bears within Ursinae, perhaps aside from the sloth bear. Two lineages evolved from U. minimus: the black bears (including the sun bear, the Asian black bear, and the American black bear); and the brown bears (which includes the polar bear). Modern brown bears evolved from U. minimus via Ursus etruscus, which itself is ancestral to the extinct Pleistocene cave bear. Species of Ursinae have migrated repeatedly into North America from Eurasia as early as 4 Mya during the early Pliocene. The polar bear is the most recently evolved species and descended from a population of brown bears that became isolated in northern latitudes by glaciation 400,000 years ago.
Phylogeny
The relationship of the bear family with other carnivorans is shown in the following phylogenetic tree, which is based on the molecular phylogenetic analysis of six genes in Flynn (2005) with the musteloids updated following the multigene analysis of Law et al. (2018).
Note that although they are called "bears" in some languages, red pandas and raccoons and their close relatives are not bears, but rather musteloids.
There are two phylogenetic hypotheses on the relationships among extant and fossil bear species. One is all species of bears are classified in seven subfamilies as adopted here and related articles: Amphicynodontinae, Hemicyoninae, Ursavinae, Agriotheriinae, Ailuropodinae, Tremarctinae, and Ursinae. Below is a cladogram of the subfamilies of bears after McLellan and Reiner (1992) and Qiu et al.. (2014): | Bear | Wikipedia | 493 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The second alternative phylogenetic hypothesis was implemented by McKenna et al. (1997) to classify all the bear species into the superfamily Ursoidea, with Hemicyoninae and Agriotheriinae being classified in the family "Hemicyonidae". Amphicynodontinae under this classification were classified as stem-pinnipeds in the superfamily Phocoidea. In the McKenna and Bell classification, both bears and pinnipeds are in a parvorder of carnivoran mammals known as Ursida, along with the extinct bear dogs of the family Amphicyonidae. Below is the cladogram based on McKenna and Bell (1997) classification:
Physical characteristics
Size
The bear family includes the most massive extant terrestrial members of the order Carnivora. The polar bear is considered to be the largest extant species, with adult males weighing and measuring in total length. The smallest species is the sun bear, which ranges in weight and in length. Prehistoric North and South American short-faced bears were the largest species known to have lived. The latter estimated to have weighed and stood tall. Body weight varies throughout the year in bears of temperate and arctic climates, as they build up fat reserves in the summer and autumn and lose weight during the winter.
Morphology
Bears are generally bulky and robust animals with short tails. They are sexually dimorphic with regard to size, with males typically being larger. Larger species tend to show increased levels of sexual dimorphism in comparison to smaller species. Relying as they do on strength rather than speed, bears have relatively short limbs with thick bones to support their bulk. The shoulder blades and the pelvis are correspondingly massive. The limbs are much straighter than those of the big cats as there is no need for them to flex in the same way due to the differences in their gait. The strong forelimbs are used to catch prey, excavate dens, dig out burrowing animals, turn over rocks and logs to locate prey, and club large creatures. | Bear | Wikipedia | 413 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Unlike most other land carnivorans, bears are plantigrade. They distribute their weight toward the hind feet, which makes them look lumbering when they walk. They are capable of bursts of speed but soon tire, and as a result mostly rely on ambush rather than the chase. Bears can stand on their hind feet and sit up straight with remarkable balance. Their front paws are flexible enough to grasp fruit and leaves. Bears' non-retractable claws are used for digging, climbing, tearing, and catching prey. The claws on the front feet are larger than those on the back and may be a hindrance when climbing trees; black bears are the most arboreal of the bears, and have the shortest claws. Pandas are unique in having a bony extension on the wrist of the front feet which acts as a thumb, and is used for gripping bamboo shoots as the animals feed.
Most mammals have agouti hair, with each individual hair shaft having bands of color corresponding to two different types of melanin pigment. Bears however have a single type of melanin and the hairs have a single color throughout their length, apart from the tip which is sometimes a different shade. The coat consists of long guard hairs, which form a protective shaggy covering, and short dense hairs which form an insulating layer trapping air close to the skin. The shaggy coat helps maintain body heat during winter hibernation and is shed in the spring leaving a shorter summer coat. Polar bears have hollow, translucent guard hairs which gain heat from the sun and conduct it to the dark-colored skin below. They have a thick layer of blubber for extra insulation, and the soles of their feet have a dense pad of fur. While bears tend to be uniform in color, some species may have markings on the chest or face and the giant panda has a bold black-and-white pelage. | Bear | Wikipedia | 380 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Bears have small rounded ears so as to minimize heat loss, but neither their hearing or sight are particularly acute. Unlike many other carnivorans they have color vision, perhaps to help them distinguish ripe nuts and fruits. They are unique among carnivorans in not having touch-sensitive whiskers on the muzzle; however, they have an excellent sense of smell, better than that of the dog, or possibly any other mammal. They use smell for signalling to each other (either to warn off rivals or detect mates) and for finding food. Smell is the principal sense used by bears to locate most of their food, and they have excellent memories which helps them to relocate places where they have found food before.
The skulls of bears are massive, providing anchorage for the powerful masseter and temporal jaw muscles. The canine teeth are large but mostly used for display, and the molar teeth flat and crushing. Unlike most other members of the Carnivora, bears have relatively undeveloped carnassial teeth, and their teeth are adapted for a diet that includes a significant amount of vegetable matter. Considerable variation occurs in dental formula even within a given species. This may indicate bears are still in the process of evolving from a mainly meat-eating diet to a predominantly herbivorous one. Polar bears appear to have secondarily re-evolved carnassial-like cheek teeth, as their diets have switched back towards carnivory. Sloth bears lack lower central incisors and use their protrusible lips for sucking up the termites on which they feed. The general dental formula for living bears is: . The structure of the larynx of bears appears to be the most basal of the caniforms. They possess air pouches connected to the pharynx which may amplify their vocalizations.
Bears have a fairly simple digestive system typical for carnivorans, with a single stomach, short undifferentiated intestines and no cecum. Even the herbivorous giant panda still has the digestive system of a carnivore, as well as carnivore-specific genes. Its ability to digest cellulose is ascribed to the microbes in its gut. Bears must spend much of their time feeding in order to gain enough nutrition from foliage. The panda, in particular, spends 12–15 hours a day feeding.
Distribution and habitat | Bear | Wikipedia | 488 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Extant bears are found in sixty countries primarily in the Northern Hemisphere and are concentrated in Asia, North America, and Europe. An exception is the spectacled bear; native to South America, it inhabits the Andean region. The sun bear's range extends below the equator in Southeast Asia. The Atlas bear, a subspecies of the brown bear was distributed in North Africa from Morocco to Libya, but it became extinct around the 1870s.
The most widespread species is the brown bear, which occurs from Western Europe eastwards through Asia to the western areas of North America. The American black bear is restricted to North America, and the polar bear is restricted to the Arctic Ocean. All the remaining species of bear are Asian. They occur in a range of habitats which include tropical lowland rainforest, both coniferous and broadleaf forests, prairies, steppes, montane grassland, alpine scree slopes, Arctic tundra and in the case of the polar bear, ice floes. Bears may dig their dens in hillsides or use caves, hollow logs and dense vegetation for shelter.
Behavior and ecology
Brown and American black bears are generally diurnal, meaning that they are active for the most part during the day, though they may forage substantially by night. Other species may be nocturnal, active at night, though female sloth bears with cubs may feed more at daytime to avoid competition from conspecifics and nocturnal predators. Bears are overwhelmingly solitary and are considered to be the most asocial of all the Carnivora. The only times bears are encountered in groups are mothers with young or occasional seasonal bounties of rich food (such as salmon runs). Fights between males can occur and older individuals may have extensive scarring, which suggests that maintaining dominance can be intense. With their acute sense of smell, bears can locate carcasses from several kilometres away. They use olfaction to locate other foods, encounter mates, avoid rivals and recognize their cubs.
Feeding | Bear | Wikipedia | 398 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Most bears are opportunistic omnivores and consume more plant than animal matter, and appear to have evolved from an ancestor which was a low-protein macronutrient omnivore. They eat anything from leaves, roots, and berries to insects, carrion, fresh meat, and fish, and have digestive systems and teeth adapted to such a diet. At the extremes are the almost entirely herbivorous giant panda and the mostly carnivorous polar bear. However, all bears feed on any food source that becomes seasonally available. For example, Asiatic black bears in Taiwan consume large numbers of acorns when these are most common, and switch to ungulates at other times of the year.
When foraging for plants, bears choose to eat them at the stage when they are at their most nutritious and digestible, typically avoiding older grasses, sedges and leaves. Hence, in more northern temperate areas, browsing and grazing is more common early in spring and later becomes more restricted. Knowing when plants are ripe for eating is a learned behavior. Berries may be foraged in bushes or at the tops of trees, and bears try to maximize the number of berries consumed versus foliage. In autumn, some bear species forage large amounts of naturally fermented fruits, which affects their behavior. Smaller bears climb trees to obtain mast (edible reproductive parts, such as acorns). Such masts can be very important to the diets of these species, and mast failures may result in long-range movements by bears looking for alternative food sources. Brown bears, with their powerful digging abilities, commonly eat roots. The panda's diet is over 99% bamboo, of 30 different species. Its strong jaws are adapted for crushing the tough stems of these plants, though they prefer to eat the more nutritious leaves. Bromeliads can make up to 50% of the diet of the spectacled bear, which also has strong jaws to bite them open. | Bear | Wikipedia | 404 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The sloth bear is not as specialized as polar bears and the panda, has lost several front teeth usually seen in bears, and developed a long, suctioning tongue to feed on the ants, termites, and other burrowing insects. At certain times of the year, these insects can make up 90% of their diets. Some individuals become addicted to sweets in garbage inside towns where tourism-related waste is generated throughout the year. Some species may raid the nests of wasps and bees for the honey and immature insects, in spite of stinging from the adults. Sun bears use their long tongues to lick up both insects and honey. Fish are an important source of food for some species, and brown bears in particular gather in large numbers at salmon runs. Typically, a bear plunges into the water and seizes a fish with its jaws or front paws. The preferred parts to eat are the brain and eggs. Small burrowing mammals like rodents may be dug out and eaten.
The brown bear and both species of black bears sometimes take large ungulates, such as deer and bovids, mostly the young and weak. These animals may be taken by a short rush and ambush, though hiding young may be sniffed out and pounced on. The polar bear mainly preys on seals, stalking them from the ice or breaking into their dens. They primarily eat the highly digestible blubber. Large mammalian prey is typically killed with raw strength, including bites and paw swipes, and bears do not display the specialized killing methods of felids and canids. Predatory behavior in bears is typically taught to the young by the mother.
Bears are prolific scavengers and kleptoparasites, stealing food caches from rodents, and carcasses from other predators. For hibernating species, weight gain is important as it provides nourishment during winter dormancy. A brown bear can eat of food and gain of fat a day prior to entering its den.
Communication | Bear | Wikipedia | 403 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Bears produce a number of vocal and non-vocal sounds. Tongue-clicking, grunting or chuffing many be made in cordial situations, such as between mothers and cubs or courting couples, while moaning, huffing, snorting or blowing air is made when an individual is stressed. Barking is produced during times of alarm, excitement or to give away the animal's position. Warning sounds include jaw-clicking and lip-popping, while teeth-chatters, bellows, growls, roars and pulsing sounds are made in aggressive encounters. Cubs may squeal, bawl, bleat or scream when in distress and make motor-like humming when comfortable or nursing.
Bears sometimes communicate with visual displays such as standing upright, which exaggerates the individual's size. The chest markings of some species may add to this intimidating display. Staring is an aggressive act and the facial markings of spectacled bears and giant pandas may help draw attention to the eyes during agonistic encounters. Individuals may approach each other by stiff-legged walking with the head lowered. Dominance between bears is asserted by making a frontal orientation, showing the canine teeth, muzzle twisting and neck stretching. A subordinate may respond with a lateral orientation, by turning away and dropping the head and by sitting or lying down.
Bears also communicate with their scent by urinating on or rubbing against trees and other objects. This is usually accompanied by clawing and biting the object. Bark may be spread around to draw attention to the marking post. Pandas establish territories by marking objects with urine and a waxy substance from their anal glands. Polar bears leave behind their scent in their tracks which allow individuals to keep track of one another in the vast Arctic wilderness.
Reproduction and development
The mating system of bears has variously been described as a form of polygyny, promiscuity and serial monogamy. During the breeding season, males take notice of females in their vicinity and females become more tolerant of males. A male bear may visit a female continuously over a period of several days or weeks, depending on the species, to test her reproductive state. During this time period, males try to prevent rivals from interacting with their mate. Courtship may be brief, although in some Asian species, courting pairs may engage in wrestling, hugging, mock fighting and vocalizing. Ovulation is induced by mating, which can last up to 30 minutes depending on the species. | Bear | Wikipedia | 492 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Gestation typically lasts six to nine months, including delayed implantation, and litter size numbers up to four cubs. Giant pandas may give birth to twins but they can only suckle one young and the other is left to die. In northern living species, birth takes place during winter dormancy. Cubs are born blind and helpless with at most a thin layer of hair, relying on their mother for warmth. The milk of the female bear is rich in fat and antibodies and cubs may suckle for up to a year after they are born. By two to three months, cubs can follow their mother outside the den. They usually follow her on foot, but sloth bear cubs may ride on their mother's back. Male bears play no role in raising young. Infanticide, where an adult male kills the cubs of another, has been recorded in polar bears, brown bears and American black bears but not in other species. Males kill young to bring the female into estrus. Cubs may flee and the mother defends them even at the cost of her life.
In some species, offspring may become independent around the next spring, though some may stay until the female successfully mates again. Bears reach sexual maturity shortly after they disperse; at around three to six years depending on the species. Male Alaskan brown bears and polar bears may continue to grow until they are 11 years old. Lifespan may also vary between species. The brown bear can live an average of 25 years.
Hibernation
Bears of northern regions, including the American black bear and the grizzly bear, hibernate in the winter. During hibernation, the bear's metabolism slows down, its body temperature decreases slightly, and its heart rate slows from a normal value of 55 to just 9 beats per minute. Bears normally do not wake during their hibernation, and can go the entire period without eating, drinking, urinating, or defecating. A fecal plug is formed in the colon, and is expelled when the bear wakes in the spring. If they have stored enough body fat, their muscles remain in good condition, and their protein maintenance requirements are met from recycling waste urea. Female bears give birth during the hibernation period, and are roused when doing so.
Mortality | Bear | Wikipedia | 459 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Bears do not have many predators. The most important are humans, and as they started cultivating crops, they increasingly came in conflict with the bears that raided them. Since the invention of firearms, people have been able to kill bears with greater ease. Felids like the tiger may also prey on bears, particularly cubs, which may also be threatened by canids.
Bears are parasitized by eighty species of parasites, including single-celled protozoans and gastro-intestinal worms, and nematodes and flukes in their heart, liver, lungs and bloodstream. Externally, they have ticks, fleas and lice. A study of American black bears found seventeen species of endoparasite including the protozoan Sarcocystis, the parasitic worm Diphyllobothrium mansonoides, and the nematodes Dirofilaria immitis, Capillaria aerophila, Physaloptera sp., Strongyloides sp. and others. Of these, D. mansonoides and adult C. aerophila were causing pathological symptoms. By contrast, polar bears have few parasites; many parasitic species need a secondary, usually terrestrial, host, and the polar bear's life style is such that few alternative hosts exist in their environment. The protozoan Toxoplasma gondii has been found in polar bears, and the nematode Trichinella nativa can cause a serious infection and decline in older polar bears. Bears in North America are sometimes infected by a Morbillivirus similar to the canine distemper virus. They are susceptible to infectious canine hepatitis (CAV-1), with free-living black bears dying rapidly of encephalitis and hepatitis.
Relationship with humans
Conservation
In modern times, bears have come under pressure through encroachment on their habitats and illegal trade in bear parts, including the Asian bile bear market, though hunting is now banned, largely replaced by farming. The IUCN lists six bear species as vulnerable; even the two least concern species, the brown bear and the American black bear, are at risk of extirpation in certain areas. In general, these two species inhabit remote areas with little interaction with humans, and the main non-natural causes of mortality are hunting, trapping, road-kill and depredation. | Bear | Wikipedia | 477 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Laws have been passed in many areas of the world to protect bears from habitat destruction. Public perception of bears is often positive, as people identify with bears due to their omnivorous diets, their ability to stand on two legs, and their symbolic importance. Support for bear protection is widespread, at least in more affluent societies. The giant panda has become a worldwide symbol of conservation. The Sichuan Giant Panda Sanctuaries, which are home to around 30% of the wild panda population, gained a UNESCO World Heritage Site designation in 2006. Where bears raid crops or attack livestock, they may come into conflict with humans. In poorer rural regions, attitudes may be more shaped by the dangers posed by bears, and the economic costs they cause to farmers and ranchers.
Attacks
Several bear species are dangerous to humans, especially in areas where they have become used to people; elsewhere, they generally avoid humans. Injuries caused by bears are rare, but are widely reported. Bears may attack humans in response to being startled, in defense of young or food, or even for predatory reasons.
Entertainment, hunting, food and folk medicine
Bears in captivity have for centuries been used for entertainment. They have been trained to dance, and were kept for baiting in Europe from at least the 16th century. There were five bear-baiting gardens in Southwark, London, at that time; archaeological remains of three of these have survived. Across Europe, nomadic Romani bear handlers called Ursari lived by busking with their bears from the 12th century.
Bears have been hunted for sport, food, and folk medicine. Their meat is dark and stringy, like a tough cut of beef. In Cantonese cuisine, bear paws are considered a delicacy. Bear meat should be cooked thoroughly, as it can be infected with the parasite Trichinella spiralis.
The peoples of eastern Asia use bears' body parts and secretions (notably their gallbladders and bile) as part of traditional Chinese medicine. More than 12,000 bears are thought to be kept on farms in China, Vietnam, and South Korea for the production of bile. Trade in bear products is prohibited under CITES, but bear bile has been detected in shampoos, wine and herbal medicines sold in Canada, the United States and Australia.
Cultural depictions | Bear | Wikipedia | 460 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Bears have been popular subjects in art, literature, folklore and mythology. The image of the mother bear was prevalent throughout societies in North America and Eurasia, based on the female's devotion and protection of her cubs. In many Native American cultures, the bear is a symbol of rebirth because of its hibernation and re-emergence. A widespread belief among cultures of North America and northern Asia associated bears with shaman; this may be based on the solitary nature of both. Bears have thus been thought to predict the future and shaman were believed to have been capable of transforming into bears.
There is evidence of prehistoric bear worship, though this is disputed by archaeologists. It is possible that bear worship existed in early Chinese and Ainu cultures. The prehistoric Finns, Siberian peoples and more recently Koreans considered the bear as the spirit of their forefathers. Artio (Dea Artio in the Gallo-Roman religion) was a Celtic bear goddess. Evidence of her worship has notably been found at Bern, itself named for the bear. Her name is derived from the Celtic word for "bear", artos. In ancient Greece, the archaic cult of Artemis in bear form survived into Classical times at Brauron, where young Athenian girls passed an initiation rite as arktoi "she bears".
The constellations of Ursa Major and Ursa Minor, the great and little bears, are named for their supposed resemblance to bears, from the time of Ptolemy. The nearby star Arcturus means "guardian of the bear", as if it were watching the two constellations. Ursa Major has been associated with a bear for as much as 13,000 years since Paleolithic times, in the widespread Cosmic Hunt myths. These are found on both sides of the Bering land bridge, which was lost to the sea some 11,000 years ago. | Bear | Wikipedia | 372 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
Bears are popular in children's stories, including Winnie the Pooh, Paddington Bear, Gentle Ben and "The Brown Bear of Norway". An early version of "Goldilocks and the Three Bears", was published as "The Three Bears" in 1837 by Robert Southey, many times retold, and illustrated in 1918 by Arthur Rackham. The Hanna-Barbera character Yogi Bear has appeared in numerous comic books, animated television shows and films. The Care Bears began as greeting cards in 1982, and were featured as toys, on clothing and in film. Around the world, many children—and some adults—have teddy bears, stuffed toys in the form of bears, named after the American statesman Theodore Roosevelt when in 1902 he had refused to shoot an American black bear tied to a tree.
Bears, like other animals, may symbolize nations. The Russian Bear has been a common national personification for Russia from the 16th century onward. Smokey Bear has become a part of American culture since his introduction in 1944, with his message "Only you can prevent forest fires".
Organizations
The International Association for Bear Research & Management, also known as the International Bear Association, and the Bear Specialist Group of the Species Survival Commission, a part of the International Union for Conservation of Nature focus on the natural history, management, and conservation of bears. Bear Trust International works for wild bears and other wildlife through four core program initiatives, namely Conservation Education, Wild Bear Research, Wild Bear Management, and Habitat Conservation.
Specialty organizations for each of the eight species of bears worldwide include:
Vital Ground, for the brown bear
Moon Bears, for the Asiatic black bear
Black Bear Conservation Coalition, for the North American black bear
Polar Bears International, for the polar bear
Bornean Sun Bear Conservation Centre, for the sun bear
Wildlife SOS, for the sloth bear
Andean Bear Conservation Project, for the Andean bear
Chengdu Research Base of Giant Panda Breeding, for the giant panda | Bear | Wikipedia | 396 | 4400 | https://en.wikipedia.org/wiki/Bear | Biology and health sciences | Carnivora | null |
The bald eagle (Haliaeetus leucocephalus) is a bird of prey found in North America. A sea eagle, it has two known subspecies and forms a species pair with the white-tailed eagle (Haliaeetus albicilla), which occupies the same niche as the bald eagle in the Palearctic. Its range includes most of Canada and Alaska, all of the contiguous United States, and northern Mexico. It is found near large bodies of open water with an abundant food supply and old-growth trees for nesting.
The bald eagle is an opportunistic feeder which subsists mainly on fish, which it swoops down upon and snatches from the water with its talons. It builds the largest nest of any North American bird and the largest tree nests ever recorded for any animal species, up to deep, wide, and in weight. Sexual maturity is attained at the age of four to five years.
Bald eagles are not bald; the name derives from an older meaning of the word, "white headed". The adult is mainly brown with a white head and tail. The sexes are identical in plumage, but females are about 25 percent larger than males. The yellow beak is large and hooked. The plumage of the immature is brown.
The bald eagle is the national symbol of the United States and appears on its seal. In the late 20th century it was on the brink of extirpation in the contiguous United States. Populations have since recovered, and the species' status was upgraded from "endangered" to "threatened" in 1995 and removed from the list altogether in 2007.
Taxonomy
The bald eagle is placed in the genus Haliaeetus (sea eagles), and gets both its common and specific scientific names from the distinctive appearance of the adult's head. Bald in the English name is from an older usage meaning "having white on the face or head" rather than "hairless", referring to the white head feathers contrasting with the darker body. The genus name is Neo-Latin: Haliaeetus (from the ), and the specific name, leucocephalus, is Latinized () and ().
The bald eagle was one of the many species originally described by Carl Linnaeus in his 18th-century work Systema Naturae, under the name Falco leucocephalus. | Bald eagle | Wikipedia | 482 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
There are two recognized subspecies of bald eagle:
H. l. leucocephalus (Linnaeus, 1766) is the nominate subspecies. It is found in the southern United States and Baja California Peninsula.
H. l. washingtoniensis (Audubon, 1827), synonym H. l. alascanus (Townsend, 1897), the northern subspecies, is larger than southern nominate leucocephalus. It is found in the northern United States, Canada and Alaska.
The bald eagle forms a species pair with the white-tailed eagle of Eurasia. This species pair consists of a white-headed and a tan-headed species of roughly equal size; the white-tailed eagle also has overall somewhat paler brown body plumage. The two species fill the same ecological niche in their respective ranges. The pair diverged from other sea eagles at the beginning of the Early Miocene (c. 10 Ma BP) at the latest, but possibly as early as the Early/Middle Oligocene, 28 Ma BP, if the most ancient fossil record is correctly assigned to this genus.
Description
The plumage of an adult bald eagle is evenly dark brown with a white head and tail. The tail is moderately long and slightly wedge-shaped. Males and females are identical in plumage coloration, but sexual dimorphism is evident in the species, in that females are 25% larger than males. The beak, feet and irises are bright yellow. The legs are feather-free, and the toes are short and powerful with large talons. The highly developed talon of the hind toe is used to pierce the vital areas of prey while it is held immobile by the front toes. The beak is large and hooked, with a yellow cere. The adult bald eagle is unmistakable in its native range. The closely related African fish eagle (Haliaeetus vocifer) (from far outside the bald eagle's range) also has a brown body (albeit of somewhat more rufous hue), white head and tail, but differs from the bald eagle in having a white chest and black tip to the bill. | Bald eagle | Wikipedia | 427 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The plumage of the immature is a dark brown overlaid with messy white streaking until the fifth (rarely fourth, very rarely third) year, when it reaches sexual maturity. Immature bald eagles are distinguishable from the golden eagle (Aquila chrysaetos), the only other very large, non-vulturine raptorial bird in North America, in that the former has a larger, more protruding head with a larger beak, straighter edged wings which are held flat (not slightly raised) and with a stiffer wing beat and feathers which do not completely cover the legs. When seen well, the golden eagle is distinctive in plumage with a more solid warm brown color than an immature bald eagle, with a reddish-golden patch to its nape and (in immature birds) a highly contrasting set of white squares on the wing.
The bald eagle has sometimes been considered the largest true raptor (accipitrid) in North America. The only larger species of raptor-like bird is the California condor (Gymnogyps californianus), a New World vulture which today is not generally considered a taxonomic ally of true accipitrids. However, the golden eagle, averaging and in wing chord length in its American race (Aquila chrysaetos canadensis), is merely lighter in mean body mass and exceeds the bald eagle in mean wing chord length by around . Additionally, the bald eagle's close cousins, the relatively longer-winged but shorter-tailed white-tailed eagle and the overall larger Steller's sea eagle (Haliaeetus pelagicus), may, rarely, wander to coastal Alaska from Asia.
The bald eagle has a body length of . Typical wingspan is between and mass is normally between . Females are about 25% larger than males, averaging as much as , and against the males' average weight of . | Bald eagle | Wikipedia | 382 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The size of the bird varies by location and generally corresponds with Bergmann's rule: the species increases in size further away from the equator and the tropics. For example, eagles from South Carolina average in mass and in wingspan, smaller than their northern counterparts. One field guide in Florida listed similarly small sizes for bald eagles there, at about . Of intermediate size, 117 migrant bald eagles in Glacier National Park were found to average but this was mostly (possibly post-dispersal) juvenile eagles, with 6 adults here averaging . Wintering eagles in Arizona (winter weights are usually the highest of the year since, like many raptors, they spend the highest percentage of time foraging during winter) were found to average .
The largest eagles are from Alaska, where large females may weigh more than and span across the wings. A survey of adult weights in Alaska showed that females there weighed on average , respectively, and males weighed against immatures which averaged and in the two sexes. An Alaskan adult female eagle that was considered outsized weighed some . R.S. Palmer listed a record from 1876 in Wyoming County, New York of an enormous adult bald eagle that was shot and reportedly scaled . Among standard linear measurements, the wing chord is , the tail is long, and the tarsus is . The culmen reportedly ranges from , while the measurement from the gape to the tip of the bill is . The bill size is unusually variable: Alaskan eagles can have up to twice the bill length of birds from the southern United States (Georgia, Louisiana, Florida), with means including both sexes of and in culmen length, respectively, from these two areas.
The call consists of weak staccato, chirping whistles, kleek kik ik ik ik, somewhat similar in cadence to a gull's call. The calls of young birds tend to be more harsh and shrill than those of adults.
Range | Bald eagle | Wikipedia | 390 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The bald eagle's natural range covers most of North America, including most of Canada, all of the continental United States, and northern Mexico. It is the only sea eagle endemic to North America. Occupying varied habitats from the bayous of Louisiana to the Sonoran Desert and the eastern deciduous forests of Quebec and New England, northern birds are migratory, while southern birds are resident, remaining on their breeding territory all year. At minimum population, in the 1950s, it was largely restricted to Alaska, the Aleutian Islands, northern and eastern Canada, and Florida. From 1966 to 2015 bald eagle numbers increased substantially throughout its winter and breeding ranges, and as of 2018 the species nests in every continental state and province in the United States and Canada.
The majority of bald eagles in Canada are found along the British Columbia coast while large populations are found in the forests of Alberta, Saskatchewan, Manitoba and Ontario. Bald eagles also congregate in certain locations in winter. From November until February, one to two thousand birds winter in Squamish, British Columbia, about halfway between Vancouver and Whistler. In March 2024, bald eagles were found nesting in Toronto for the first time. The birds primarily gather along the Squamish and Cheakamus Rivers, attracted by the salmon spawning in the area. Similar congregations of wintering bald eagles at open lakes and rivers, wherein fish are readily available for hunting or scavenging, are observed in the northern United States.
It has occurred as a vagrant twice in Ireland; a juvenile was shot illegally in Fermanagh on January 11, 1973 (misidentified at first as a white-tailed eagle), and an exhausted juvenile was captured near Castleisland, County Kerry on November 15, 1987. There is also a record of it from Llyn Coron, Anglesey, in the United Kingdom, from October 17, 1978; the provenance of this individual eagle has remained in dispute.
Habitat
The bald eagle occurs during its breeding season in virtually any kind of American wetland habitat such as seacoasts, rivers, large lakes or marshes or other large bodies of open water with an abundance of fish. Studies have shown a preference for bodies of water with a circumference greater than , and lakes with an area greater than are optimal for breeding bald eagles. | Bald eagle | Wikipedia | 468 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The bald eagle typically requires old-growth and mature stands of coniferous or hardwood trees for perching, roosting, and nesting. Tree species reportedly is less important to the eagle pair than the tree's height, composition and location. Perhaps of paramount importance for this species is an abundance of comparatively large trees surrounding the body of water. Selected trees must have good visibility, be over tall, an open structure, and proximity to prey. If nesting trees are in standing water such as in a mangrove swamp, the nest can be located fairly low, at as low as above the ground. In a more typical tree standing on dry ground, nests may be located from in height. In Chesapeake Bay, nesting trees averaged in diameter and in total height, while in Florida, the average nesting tree stands high and is in diameter. Trees used for nesting in the Greater Yellowstone area average high. Trees or forest used for nesting should have a canopy cover of no more than 60%, and no less than 20%, and be in close proximity to water. Most nests have been found within of open water. The greatest distance from open water recorded for a bald eagle nest was over , in Florida.
Bald eagle nests are often very large in order to compensate for size of the birds. The largest recorded nest was found in Florida in 1963, and was measured at nearly 10 feet wide and 20 feet deep.
In Florida, nesting habitats often consist of Mangrove swamps, the shorelines of lakes and rivers, pinelands, seasonally flooded flatwoods, hardwood swamps, and open prairies and pastureland with scattered tall trees. Favored nesting trees in Florida are slash pines (Pinus elliottii), longleaf pines (P. palustris), loblolly pines (P. taeda) and cypress trees, but for the southern coastal areas where mangroves are usually used. In Wyoming, groves of mature cottonwoods or tall pines found along streams and rivers are typical bald eagle nesting habitats. Wyoming eagles may inhabit habitat types ranging from large, old-growth stands of ponderosa pines (Pinus ponderosa) to narrow strips of riparian trees surrounded by rangeland. In Southeast Alaska, Sitka spruce (Picea sitchensis) provided 78% of the nesting trees used by eagles, followed by hemlocks (Tsuga) at 20%. Increasingly, eagles nest in human-made reservoirs stocked with fish. | Bald eagle | Wikipedia | 487 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The bald eagle is usually quite sensitive to human activity while nesting, and is found most commonly in areas with minimal human disturbance. It chooses sites more than from low-density human disturbance and more than from medium- to high-density human disturbance. However, bald eagles will occasionally nest in large estuaries or secluded groves within major cities, such as Hardtack Island on the Willamette River in Portland, Oregon or John Heinz National Wildlife Refuge at Tinicum in Philadelphia, Pennsylvania, which are surrounded by a great quantity of human activity. Even more contrary to the usual sensitivity to disturbance, a family of bald eagles moved to the Harlem neighborhood in New York City in 2010.
While wintering, bald eagles tend to be less habitat and disturbance sensitive. They will commonly congregate at spots with plentiful perches and waters with plentiful prey and (in northern climes) partially unfrozen waters. Alternately, non-breeding or wintering bald eagles, particularly in areas with a lack of human disturbance, spend their time in various upland, terrestrial habitats sometimes quite far away from waterways. In the northern half of North America (especially the interior portion), this terrestrial inhabitance by bald eagles tends to be especially prevalent because unfrozen water may not be accessible. Upland wintering habitats often consist of open habitats with concentrations of medium-sized mammals, such as prairies, meadows or tundra, or open forests with regular carrion access. | Bald eagle | Wikipedia | 296 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Behavior
The bald eagle is a powerful flier, and soars on thermal convection currents. It reaches speeds of when gliding and flapping, and about while carrying fish. Its dive speed is between , though it seldom dives vertically. Regarding their flying abilities, despite being morphologically less well adapted to faster flight than golden eagles (especially during dives), the bald eagle is considered surprisingly maneuverable in flight. Bald eagles have also been recorded catching up to and then swooping under geese in flight, turning over and thrusting their talons into the other bird's breast. It is partially migratory, depending on location. If its territory has access to open water, it remains there year-round, but if the body of water freezes during the winter, making it impossible to obtain food, it migrates to the south or to the coast. A number of populations are subject to post-breeding dispersal, mainly in juveniles; Florida eagles, for example, will disperse northwards in the summer. The bald eagle selects migration routes which take advantage of thermals, updrafts, and food resources. During migration, it may ascend in a thermal and then glide down, or may ascend in updrafts created by the wind against a cliff or other terrain. Migration generally takes place during the daytime, usually between the local hours of 8:00 a.m. and 6:00 p.m., when thermals are produced by the sun.
Diet and feeding
The bald eagle is an opportunistic carnivore with the capacity to consume a great variety of prey. Fish often comprise most of the eagle's diet throughout their range. In 20 food habit studies across the species' range, fish comprised 56% of the diet of nesting eagles, birds 28%, mammals 14% and other prey 2%. More than 400 species are known to be included in the bald eagle's prey spectrum, far more than its ecological equivalent in the Old World, the white-tailed eagle, is known to take. Despite its considerably lower population, the bald eagle may come in second amongst all North American accipitrids, slightly behind only the red-tailed hawk, in number of prey species recorded.
Behavior | Bald eagle | Wikipedia | 452 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
To hunt fish, the eagle swoops down over the water and snatches the fish out of the water with its talons. They eat by holding the fish in one claw and tearing the flesh with the other. Eagles have structures on their toes called spicules that allow them to grasp fish. Osprey also has this adaptation. Bird prey may occasionally be attacked in flight, with prey up to the size of Canada geese attacked and killed in mid-air. It has been estimated that the bald eagle's gripping power (pounds by square inch) is ten times greater than that of a human. Bald eagles can fly with fish at least equal to their own weight, but if the fish is too heavy to lift, the eagle may be dragged into the water. Bald eagles can swim, but in some cases, they drag their catch ashore with their talons. Still, some eagles drown or succumb to hypothermia. Many sources claim that bald eagles, like all large eagles, cannot normally take flight carrying prey more than half of their own weight unless aided by favorable wind conditions. On numerous occasions, when large prey such as large fish including mature salmon or geese are attacked, eagles have been seen to make contact and then drag the prey in a strenuously labored, low flight over the water to a bank, where they then finish off and dismember the prey. When food is abundant, an eagle can gorge itself by storing up to of food in a pouch in the throat called a crop. Gorging allows the bird to fast for several days if food becomes unavailable. Occasionally, bald eagles may hunt cooperatively when confronting prey, especially relatively large prey such as jackrabbits or herons, with one bird distracting potential prey, while the other comes behind it in order to ambush it. While hunting waterfowl, bald eagles repeatedly fly at a target and cause it to dive repeatedly, hoping to exhaust the victim so it can be caught (white-tailed eagles have been recorded hunting waterfowl in the same way). When hunting concentrated prey, a successful catch often results in the hunting eagle being pursued by other eagles and needing to find an isolated perch for consumption if it is able to carry it away successfully. | Bald eagle | Wikipedia | 458 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
They obtain much of their food as carrion or via a practice known as kleptoparasitism, by which they steal prey away from other predators. Due to their dietary habits, bald eagles are frequently viewed in a negative light by humans. Thanks to their superior foraging ability and experience, adults are generally more likely to hunt live prey than immature eagles, which often obtain their food from scavenging. They are not very selective about the condition or origin, whether provided by humans, other animals, auto accidents or natural causes, of a carcass's presence, but will avoid eating carrion where disturbances from humans are a regular occurrence. They will scavenge carcasses up to the size of whales, though carcasses of ungulates and large fish are seemingly preferred. Congregated wintering waterfowl are frequently exploited for carcasses to scavenge by immature eagles in harsh winter weather. Bald eagles also may sometimes feed on material scavenged or stolen from campsites and picnics, as well as garbage dumps (dump usage is habitual mainly in Alaska) and fish-processing plants.
Fish
In Southeast Alaska, fish comprise approximately 66% of the year-round diet of bald eagles and 78% of the prey brought to the nest by the parents. Eagles living in the Columbia River Estuary in Oregon were found to rely on fish for 90% of their dietary intake. At least 100 species of fish have been recorded in the bald eagle's diet. From observation in the Columbia River, 58% of the fish were caught alive by the eagle, 24% were scavenged as carcasses and 18% were pirated away from other animals. | Bald eagle | Wikipedia | 344 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
In the Pacific Northwest, spawning trout and salmon provide most of the bald eagles' diet from late summer throughout fall. Though bald eagles occasionally catch live salmon, they usually scavenge spawned salmon carcass. Southeast Alaskan eagles largely prey on pink salmon (Oncorhynchus gorbuscha), coho salmon (O. kisutch) and, more locally, sockeye salmon (O. nerka), with Chinook salmon (O. tshawytscha). Due to the Chinook salmon's large size ( average adult size) probably being taken only as carrion and a single carcass can attract several eagles. Also important in the estuaries and shallow coastlines of southern Alaska are Pacific herring (Clupea pallasii), Pacific sand lance (Ammodytes hexapterus) and eulachon (Thaleichthys pacificus). In Oregon's Columbia River Estuary, the most significant prey species were largescale suckers (Catostomus macrocheilus) (17.3% of the prey selected there), American shad (Alosa sapidissima; 13%) and common carp (Cyprinus carpio; 10.8%). Eagles living in the Chesapeake Bay in Maryland were found to subsist largely on American gizzard shad (Dorosoma cepedianum), threadfin shad (Dorosoma petenense) and white bass (Morone chrysops). Floridian eagles have been reported to prey on catfish, most prevalently the brown bullhead (Ameiurus nebulosus) and any species in the genus Ictalurus as well as mullet, trout, needlefish, and eels. Chain pickerels (Esox niger) and white suckers (Catostomus commersonii) are frequently taken in interior Maine. Wintering eagles on the Platte River in Nebraska preyed mainly on American gizzard shads and common carp | Bald eagle | Wikipedia | 417 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Bald eagles are also known to eat the following fish species: rainbow trout (Oncorhynchus mykiss), white catfish (Ameiurus catus), rock greenling (Hexagrammos lagocephalus), Pacific cod (Gadus macrocephalus), Atka mackerel (Pleurogrammus monopterygius), largemouth bass (Micropterus salmoides), northern pike (Esox lucius), striped bass (Morone saxatilis), dogfish shark (Squalidae.sp) and Blue walleye (Sander vitreus) | Bald eagle | Wikipedia | 130 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Fish taken by bald eagles varies in size, but bald eagles take larger fish than other piscivorous birds in North America, typically range from and prefer fish. When experimenters offered fish of different sizes in the breeding season around Lake Britton in California, fish measuring were taken 71.8% of the time by parent eagles while fish measuring were chosen only 25% of the time. At nests around Lake Superior, the remains of fish (mostly suckers) were found to average in total length. In the Columbia River estuary, most preyed on by eagles were estimated to measure less than , but larger fish between or even exceeding in length also taken especially during the non-breeding seasons. They can take fish up to at least twice their own weight, such as large mature salmons, carps, or even muskellunge (Esox masquinongy), by dragging its catch with talons and pull toward ashore. Much larger marine fish such as Pacific halibut (Hippoglossus stenolepis) and lemon sharks (Negaprion brevirostris) have been recorded among bald eagle prey though probably are only taken as young, as small, newly mature fish, or as carrion.
Benthic fishes such as catfish are usually consumed after they die and float to the surface, though while temporarily swimming in the open may be more vulnerable to predation than most fish since their eyes focus downwards. Bald eagles also regularly exploit water turbines which produce battered, stunned or dead fish easily consumed. Predators who leave behind scraps of dead fish that they kill, such as brown bears (Ursus arctos), gray wolves (Canis lupus) and red foxes (Vulpes vulpes), may be habitually followed in order to scavenge the kills secondarily. Once North Pacific salmon die off after spawning, usually local bald eagles eat salmon carcasses almost exclusively. Eagles in Washington need to consume of fish each day for survival, with adults generally consuming more than juveniles and thus reducing potential energy deficiency and increasing survival during winter.
Birds | Bald eagle | Wikipedia | 426 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Behind fish, the next most significant prey base for bald eagles are other waterbirds. The contribution of such birds to the eagle's diet is variable, depending on the quantity and availability of fish near the water's surface. Waterbirds can seasonally comprise from 7% to 80% of the prey selection for eagles in certain localities. Overall, birds are the most diverse group in the bald eagle's prey spectrum, with 200 prey species recorded.
Bird species most preferred as prey by eagles tend to be medium-sized, such as western grebes (Aechmophorus occidentalis), mallards (Anas platyrhynchos), and American coots (Fulica americana) as such prey is relatively easy for the much larger eagles to catch and fly with. American herring gull (Larus smithsonianus) are the favored avian prey species for eagles living around Lake Superior. Black ducks (Anas rubripes), common eiders (Somateria mollissima), and double-crested cormorants (Phalacrocorax auritus) are also frequently taken in coastal Maine and velvet scoter (Melanitta fusca) was dominant prey in San Miguel Island. | Bald eagle | Wikipedia | 257 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Due to easy accessibility and lack of formidable nest defense against eagles by such species, bald eagles are capable of preying on such seabirds at all ages, from eggs to mature adults, and they can effectively cull large portions of a colony. Along some portions of the North Pacific coastline, bald eagles which had historically preyed mainly kelp-dwelling fish and supplementally sea otter (Enhydra lutris) pups are now preying mainly on seabird colonies since both the fish (possibly due to overfishing) and otters (cause unknown) have had steep population declines, causing concern for seabird conservation. Because of this more extensive predation, some biologist has expressed concern that murres are heading for a "conservation collision" due to heavy eagle predation. Eagles have been confirmed to attack nocturnally active, burrow-nesting seabird species such as storm petrels and shearwaters by digging out their burrows and feeding on all animals they find inside. If a bald eagle flies close by, waterbirds will often fly away en masse, though they may seemingly ignore a perched eagle in other cases. when the birds fly away from a colony, this exposes their unprotected eggs and nestlings to scavengers such as gulls. | Bald eagle | Wikipedia | 261 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
While they usually target small to medium-sized seabirds, larger seabirds such as great black-backed gulls (Larus marinus) and northern gannets (Morus bassanus) and brown pelicans (Pelecanus occidentalis) of all ages can successfully be taken by bald eagles. Similarly, large waterbirds are occasionally killed. Geese such as wintering emperor geese (Chen canagica) and snow geese (C. caerulescens), which gather in large groups, sometimes becoming regular prey. Smaller Ross's geese (Anser rossii) are also taken, as well as large-sized Canada geese (Branta canadensis). Predation on the largest subspecies (Branta canadensis maxima) has been reported. Other large waterbird prey include common loons (Gavia immer) of all ages. Large wading birds can also fall prey to bald eagles. For the great blue herons (Ardea herodias), bald eagles are their only serious enemies of all ages. Slightly larger Sandhill cranes (Grus canadensis) can be taken as well. While adult whooping cranes (Grus americana) are too large and formidable, their chicks can fall prey to bald eagles. They even occasionally prey on adult tundra swans (Cygnus columbianus). Young trumpeter swans (Cygnus buccinator) are also taken, and an unsuccessful attack on an adult swan has been photographed.
Bald eagles have been occasionally recorded as killing other raptors. In some cases, these may be attacks of competition or kleptoparasitism on rival species but end with the consumption of the dead victims. Nine species of other accipitrids and owls are known to have been preyed upon by bald eagles. Owl prey species have ranged in size from western screech-owls (Megascops kennicotti) to snowy owls (Bubo scandiacus). Larger diurnal raptors known to have fallen victim to bald eagles have included red-tailed hawks (Buteo jamaicensis), peregrine falcons (Falco peregrinus), northern goshawks (Accipiter gentilis), ospreys (Pandion haliaetus) and black (Coragyps atratus) and turkey vultures (Cathartes aura).
Mammals | Bald eagle | Wikipedia | 489 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Mammalian preys are generally less frequently taken than fish or avian prey. However, in some regions, such as landlocked areas of North America, wintering bald eagles may become habitual predators of medium-sized mammals that occur in colonies or local concentrations, such as prairie dogs (Cynomys sp.) and jackrabbits (Lepus sp.). Bald eagles in Seedskadee National Wildlife Refuge often hunt in pair to catch cottontails, jackrabbits and prairie dogs. They can attack and prey on rabbits and hares of nearly any size, from marsh rabbits (Sylvilagus palustris) to black and white-tailed jackrabbits (Lepus californicus & L. townsendii), and Arctic hares (Lepus arcticus). In San Luis Valley, white-tailed jackrabbits can be important prey. Additionally, rodents such as montane voles (Microtus montanus), brown rats (Rattus norvegicus), and various squirrels are taken as supplementary prey. Larger rodents such as muskrats (Ondatra zibethicus), young or small adult nutrias (Myocastor coypus) and groundhogs (Marmota monax) are also preyed upon. Even American porcupines (Erethizon dorsatum) are reportedly attacked and killed. | Bald eagle | Wikipedia | 288 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Where available, seal colonies can provide a lot of food. On Protection Island, Washington, they commonly feed on harbor seal (Phoca vitulina) afterbirths, still-borns and sickly seal pups. Similarly, bald eagles in Alaska readily prey on sea otter (Enhydra lutris) pups. Small to medium-sized terrestrial mammalian carnivores can be taken infrequently. Mustelid including American martens (Martes pennanti), American minks (Neogale vison), and larger fisher cats (Pekania pennanti) are known to be hunted. Foxes are also taken, including Island foxes ( Urocyon littoralis ), Arctic foxes (Vulpes lagopus), and grey foxes (Urocyon cinereoargenteus). Although fox farmers claimed that bald eagle heavily prey on young and adult free-range Arctic fox, the predation events are sporadic. In one instance, two bald Eagles fed upon a red fox (Vulpes vulpes) that had tried to cross a frozen Delaware Lake. Other medium-sized carnivorans such as striped skunks (Mephitis mephitis), American hog-nosed skunks (Conepatus leuconotus), and common raccoons (Procyon lotor) are taken, as well as domestic cats (Felis catus) and dogs (canis familiaris).
Other wild mammalian prey include fawns of deer such as white-tailed deer (Odocoileus virginianus) and Sitka deer (Odocoileus hemionus sitkensis), which weigh around can be taken alive by bald eagles. In one instance, a bald eagle was observed carrying mule deer (Odocoileus hemionus) fawn. Additionally, Virginia opossums (Didelphis virginiana) can be preyed upon. Still, predation events are rare due to their nocturnal habits. | Bald eagle | Wikipedia | 416 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Together with the golden eagle, bald eagles are occasionally accused of preying on livestock, especially sheep (Ovis aries). There are a handful of proven cases of lamb predation, some specimens weighing up to , by bald eagles. Still, they are much less likely to attack a healthy lamb than a golden eagle. Both species prefer native, wild prey and are unlikely to cause any extensive detriment to human livelihoods. There is one case of a bald eagle killing and feeding on an adult, pregnant ewe (then joined in eating the kill by at least 3 other eagles), which, weighing on average over , is much larger than any other known prey taken by this species.
Reptiles and other prey
Supplemental prey is readily taken given the opportunity. In some areas, reptiles may become regular prey, especially in warm areas such as Florida where reptile diversity is high. Turtles are perhaps the most regularly hunted type of reptile. In coastal New Jersey, 14 of 20 studied eagle nests included remains of turtles. The main species found were common musk turtles (Sternotherus odoratus), diamondback terrapin (Malaclemys terrapin) and juvenile common snapping turtles (Chelydra serpentina). In these New Jersey nests, mainly subadult and small adults were taken, ranging in carapace length from . Similarly, many turtles were recorded in the diet in the Chesapeake Bay. In Texas, softshell turtles are the most frequently taken prey, and a large number of Barbour's map turtles are taken in Torreya State Park. Other reptilian and amphibian prey includes southern alligator lizards (Elgaria multicarinata), snakes such as garter snakes and rattlesnakes, and Greater siren (Siren lacertina).
Invertebrates are occasionally taken. In Alaska, eagles feed on sea urchins (Strongylocentrotus sp.), chitons, mussels, and crabs. Other various mollusks such as land snails, abalones, bivalves, periwinkles, blue mussels, squids, and starfishes are taken as well.
Interspecific predatory relationships | Bald eagle | Wikipedia | 445 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
When competing for food, eagles will usually dominate other fish-eaters and scavengers, aggressively displacing mammals such as coyotes (Canis latrans) and foxes, and birds such as corvids, gulls, vultures and other raptors. Occasionally, coyotes, bobcats (Lynx rufus) and domestic dogs (Canis familiaris) can displace eagles from carrion, usually less confident immature birds, as has been recorded in Maine. Bald eagles are less active, bold predators than golden eagles and get relatively more of their food as carrion and from kleptoparasitism (although it is now generally thought that golden eagles eat more carrion than was previously assumed). However, the two species are roughly equal in size, aggressiveness and physical strength and so competitions can go either way. Neither species is known to be dominant, and the outcome depends on the size and disposition of the individual eagles involved. Wintering bald and golden eagles in Utah both sometimes won conflicts, though in one recorded instance a single bald eagle successfully displaced two consecutive golden eagles from a kill.
Though bald eagles face few natural threats, an unusual attacker comes in the form of the common loon (G. immer), which is also taken by eagles as prey. While common loons normally avoid conflict, they are highly territorial and will attack predators and competitors by stabbing at them with their knife-like bill; as the range of the bald eagle has increased following conservation efforts, these interactions have been observed on several occasions, including a fatality of a bald eagle in Maine that is presumed to have come about as a result of it attacking a nest, then having a fatal puncture wound inflicted by one or both loon parents. | Bald eagle | Wikipedia | 357 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The bald eagle is thought to be much more numerous in North America than the golden eagle, with the bald species estimated to number at least 150,000 individuals, about twice as many golden eagles there are estimated to live in North America. Due to this, bald eagles often outnumber golden eagles at attractive food sources. Despite the potential for contention between these animals, in New Jersey during winter, a golden eagle and numerous bald eagles were observed to hunt snow geese alongside each other without conflict. Similarly, both eagle species have been recorded, via video-monitoring, to feed on gut piles and carcasses of white-tailed deer (Odocoileus virginianus) in remote forest clearings in the eastern Appalachian Mountains without apparent conflict. Bald eagles are frequently mobbed by smaller raptors, due to their infrequent but unpredictable tendency to hunt other birds of prey. Many bald eagles are habitual kleptoparasites, especially in winters when fish are harder to come by. They have been recorded stealing fish from other predators such as ospreys, herons and even otters. They have also been recorded opportunistically pirating birds from peregrine falcons (Falco peregrinus), prairie dogs from ferruginous hawks (Buteo regalis) and even jackrabbits from golden eagles. When they approach scavengers such as dogs, gulls or vultures at carrion sites, they often attack them in an attempt to force them to disgorge their food. Healthy adult bald eagles are not preyed upon in the wild and are thus considered apex predators.
Reproduction
Bald eagles are sexually mature at four or five years of age. When they are old enough to breed, they often return to the area where they were born. Bald eagles have high mate fidelity and generally mate for life. However, if one pair member dies or disappears, the survivor will choose a new mate. A pair that has repeatedly failed in breeding attempts may split and look for new mates. Bald eagle courtship involves elaborate, spectacular calls and flight displays by the males. The flight includes swoops, chases, and cartwheels, in which they fly high, lock talons, and free-fall, separating just before hitting the ground. Usually, a territory defended by a mature pair will be of waterside habitat. | Bald eagle | Wikipedia | 479 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Compared to most other raptors, which mostly nest in April or May, bald eagles are early breeders: nest building or reinforcing is often by mid-February, egg laying is often late February (sometimes during deep snow in the North), and incubation is usually mid-March and early May. Eggs hatch from mid-April to early May, and the young fledge from late June to early July. The nest is the largest of any bird in North America; it is used repeatedly over many years and with new material added each year may eventually be as large as deep, across and weigh . One nest in Florida was found to be deep, across, and to weigh . This nest is on record as the largest tree nest ever recorded for any animal. Usually nests are used for under five years, as they either collapse in storms or break the branches supporting them by their sheer weight. However, one nest in the Midwest was occupied continuously for at least 34 years. The nest is built of branches, usually in large trees found near water. When breeding where there are no trees, the bald eagle will nest on the ground, as has been recorded largely in areas largely isolated from terrestrial predators, such as Amchitka Island in Alaska. | Bald eagle | Wikipedia | 251 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
In Sonora, Mexico, eagles have been observed nesting on top of hecho catcuses (Pachycereus pectin-aboriginum). Nests located on cliffs and rock pinnacles have been reported historically in California, Kansas, Nevada, New Mexico and Utah, but currently are only verified to occur only in Alaska and Arizona. The eggs average about long, ranging from , and have a breadth of , ranging from . Eggs in Alaska averaged in mass, while in Saskatchewan they averaged . As with their ultimate body size, egg size tends to increase with distance from the equator. Eagles produce between one and three eggs per year, two being typical. Rarely, four eggs have been found in nests, but these may be exceptional cases of polygyny. Eagles in captivity have been capable of producing up to seven eggs. It is rare for all three chicks to successfully reach the fledgling stage. The oldest chick often bears the advantage of a larger size and louder voice, which tends to draw the parents' attention towards it. Occasionally, as is recorded in many large raptorial birds, the oldest sibling sometimes attacks and kills its younger sibling(s), especially early in the nesting period when their sizes are most different. However, nearly half of the known bald eagles produce two fledglings (more rarely three), unlike in some other "eagle" species such as some in the genus Aquila, in which a second fledgling is typically observed in less than 20% of nests, despite two eggs typically being laid. Both the male and female take turns incubating the eggs, but the female does most of the sitting. The parent not incubating will hunt for food or look for nesting material during this stage. For the first two to three weeks of the nestling period, at least one adult is at the nest almost 100% of the time. After five to six weeks, the attendance of parents usually drops off considerably (with the parents often perching in trees nearby). | Bald eagle | Wikipedia | 402 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
A young eaglet can gain up to a day, the fastest growth rate of any North American bird. The young eaglets pick up and manipulate sticks, play tug of war with each other, practice holding things in their talons, and stretch and flap their wings. By eight weeks, the eaglets are strong enough to flap their wings, lift their feet off the nest platform, and rise in the air. The young fledge at anywhere from 8 to 14 weeks of age, though will remain close to the nest and be attended to by their parents for a further 6 weeks. Juvenile eagles first start dispersing away from their parents about 8 weeks after they fledge. Variability in departure date related to effects of sex and hatching order on growth and development. For the next four years, immature eagles wander widely in search of food until they attain adult plumage and are eligible to reproduce.
Male eagles have been observed killing and cannibalizing their chicks. In 2024 at the National Conservation Training Center in West Virginia, the NCTC's Eagle Cam recorded two bald eagle chicks being attacked and devoured by their father as soon as the mother departed from the nest. The NCTC noted in its statement on the incident that such behavior "has been observed in other nests and is not uncommon in birds of prey."
On rare occasions, bald eagles have been recorded to adopt other raptor fledglings into their nests, as seen in 2017 by a pair of eagles in Shoal Harbor Migratory Bird Sanctuary near Sidney, British Columbia. The pair of eagles in question are believed to have carried a juvenile red-tailed hawk back to their nest, presumably as prey, whereupon the chick was accepted into the family by both the parents and the eagles' three nestlings. The hawk, nicknamed "Spunky" by biologists monitoring the nest, fledged successfully.
Longevity and mortality | Bald eagle | Wikipedia | 375 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The average lifespan of bald eagles in the wild is around 20 years, with the oldest confirmed one having been 38 years of age. In captivity, they often live somewhat longer. In one instance, a captive individual in New York lived for nearly 50 years. As with size, the average lifespan of an eagle population appears to be influenced by its location and access to prey. As they are no longer heavily persecuted, adult mortality is quite low. In one study of Florida eagles, adult bald eagles reportedly had 100% annual survival rate. In Prince William Sound in Alaska, adults had an annual survival rate of 88% even after the Exxon Valdez oil spill adversely affected eagles in the area. Of 1,428 individuals from across the range necropsied by National Wildlife Health Center from 1963 to 1984, 329 (23%) eagles died from trauma, primarily impact with wires and vehicles; 309 (22%) died from gunshot; 158 (11%) died from poisoning; 130 (9%) died from electrocution; 68 (5%) died from trapping; 110 (8%) from emaciation; and 31 (2%) from disease; cause of death was undetermined in 293 (20%) of cases. In this study, 68% of mortality was human-caused. Today, eagle-shooting is believed to be considerably reduced due to the species' protected status. A U.S. Fish and Wildlife Service study of 1,490 bald eagle deaths from 1986 through 2017 in Michigan found that 532 (36%) died due to being struck by cars while scavenging roadkill and 176 (12%) died due to lead poisoning from ingesting fragments of lead ammo and fishing gear present in carrion, with the proportion of both causes of death increasing significantly towards the end of the study period. | Bald eagle | Wikipedia | 375 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Most non-human-related mortality involves nestlings or eggs. Around 50% of eagles survive their first year. However, in the Chesapeake Bay area, 100% of 39 radio-tagged nestlings survived to their first year. Nestling or egg fatalities may be due to nest collapses, starvation, sibling aggression or inclement weather. Another significant cause of egg and nestling mortality is predation. Nest predators include large gulls, corvids (including ravens, crows and magpies), wolverines (Gulo gulo), fishers (Pekania pennanti), red-tailed hawks, owls, other eagles, bobcats, American black bears (Ursus americanus) and raccoons. If food access is low, parental attendance at the nest may be lower because both parents may have to forage, thus resulting in less protection. Nestlings are usually exempt from predation by terrestrial carnivores that are poor tree-climbers, but Arctic foxes (Vulpes lagopus) occasionally snatched nestlings from ground nests on Amchitka Island in Alaska before they were extirpated from the island. The bald eagle will defend its nest fiercely from all comers and has even repelled attacks from bears, having been recorded knocking a black bear out of a tree when the latter tried to climb a tree holding nestlings.
Relationship with humans
Population decline and recovery | Bald eagle | Wikipedia | 286 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Once a common sight in much of the continent, the bald eagle was severely affected in the mid-20th century by a variety of factors, among them the thinning of egg shells attributed to use of the pesticide DDT. Bald eagles, like many birds of prey, were especially affected by DDT due to biomagnification. DDT itself was not lethal to the adult bird, but it interfered with their calcium metabolism, making them either sterile or unable to lay healthy eggs; many of their eggs were too brittle to withstand the weight of a brooding adult, making it nearly impossible for them to hatch. It is estimated that in the early 18th century the bald eagle population was 300,000–500,000, but by the 1950s there were only 412 nesting pairs in the 48 contiguous states of the US.
Other factors in bald eagle population reductions were a widespread loss of suitable habitat, as well as both legal and illegal shooting. In 1930 a New York City ornithologist wrote that in the territory of Alaska in the previous 12 years approximately 70,000 bald eagles had been shot. Many of the hunters killed the bald eagles under the long-held beliefs that bald eagles grabbed young lambs and even children with their talons, yet the birds were innocent of most of these alleged acts of predation (lamb predation is rare, human predation is thought to be non-existent). Illegal shooting was described as "the leading cause of direct mortality in both adult and immature bald eagles" by the U.S. Fish and Wildlife Service in 1978. Leading causes of death in bald eagles include lead pollution, poisoning, collision with motor vehicles, and power-line electrocution. A study published in 2022 in the journal Science found that more than half of adult eagles across 38 US states suffered from lead poisoning. The primary cause is when eagles scavenge carcasses of animals shot by hunters. These are often tainted with lead shotgun pellets, rifle rounds, or fishing tackle. | Bald eagle | Wikipedia | 404 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The species was first protected in the U.S. and Canada by the 1918 Migratory Bird Treaty, later extended to all of North America. The Bald and Golden Eagle Protection Act, approved by the U.S. Congress in 1940, protected the bald eagle and the golden eagle, prohibiting commercial trapping and killing of the birds. The bald eagle was declared an endangered species in the U.S. in 1967, and amendments to the 1940 act between 1962 and 1972 further restricted commercial uses and increased penalties for violators. Perhaps most significant in the species' recovery, in 1972, DDT was banned from usage in the United States due to the fact that it inhibited the reproduction of many birds. DDT was completely banned in Canada in 1989, though its use had been highly restricted since the late 1970s.
With regulations in place and DDT banned, the eagle population rebounded. The bald eagle can be found in growing concentrations throughout the United States and Canada, particularly near large bodies of water. In the early 1980s, the estimated total population was 100,000 individuals, with 110,000–115,000 by 1992; the U.S. state with the largest resident population is Alaska, with about 40,000–50,000, with the next highest population the Canadian province of British Columbia with 20,000–30,000 in 1992. Obtaining a precise count of the bald eagle population is extremely difficult. The most recent data submitted by individual states was in 2006, when 9789 breeding pairs were reported. For some time, the stronghold breeding population of bald eagles in the lower 48 states was in Florida, where over a thousand pairs have held on while populations in other states were significantly reduced by DDT use. Today, the contiguous state with the largest number of breeding pairs of eagles is Minnesota with an estimated 1,312 pairs, surpassing Florida's most recent count of 1,166 pairs. 23, or nearly half, of the 48 contiguous states now have at least 100 breeding pairs of bald eagles. In Washington State, there were only 105 occupied nests in 1980. That number increased by about 30 per year, so that by 2005 there were 840 occupied nests. 2005 was the last year that the Washington Department of Fish and Wildlife counted occupied nests. Further population increases in Washington may be limited by the availability of late winter food, particularly salmon. | Bald eagle | Wikipedia | 472 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The bald eagle was officially removed from the U.S. federal government's list of endangered species on July 12, 1995, by the U.S. Fish & Wildlife Service, when it was reclassified from "endangered" to "threatened". On July 6, 1999, a proposal was initiated "To Remove the Bald Eagle in the Lower 48 States From the List of Endangered and Threatened Wildlife". It was de-listed on June 28, 2007. It has also been assigned a risk level of least concern category on the IUCN Red List. In the Exxon Valdez oil spill of 1989 an estimated 247 were killed in Prince William Sound, though the local population returned to its pre-spill level by 1995. In some areas, the increase in eagles has led to decreases in other bird populations and the eagles may be considered a pest.
Killing permits
In December 2016, the U.S. Fish and Wildlife Service proposed extending the permits issued to wind generation companies to allow them to kill up to 4,200 bald eagles per year without facing a penalty, four times the previous number. The permits would last 30 years, six times the previous 5-year term.
In captivity
Permits are required to keep bald eagles in captivity in the United States. Permits are primarily issued to public educational institutions, and the eagles that they show are permanently injured individuals that cannot be released to the wild. The facilities where eagles are kept must be equipped with adequate caging, as well as workers experienced in the handling and care of eagles. The bald eagle can be long-lived in captivity if well cared for, but does not breed well even under the best conditions.
In Canada and in England a license is required to keep bald eagles for falconry. Bald eagles cannot legally be kept for falconry in the United States, but a license may be issued in some jurisdictions to allow use of such eagles in birds-of-prey flight shows.
Cultural significance
The bald eagle is important in various Native American cultures and, as the national symbol of the United States, is prominent in seals and logos, coinage, postage stamps, and other items relating to the U.S. federal government. | Bald eagle | Wikipedia | 434 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
Role in Native American culture
The bald eagle is a sacred bird in some North American cultures, and its feathers, like those of the golden eagle, are central to many religious and spiritual customs among Native Americans. Eagles are considered spiritual messengers between gods and humans by some cultures. Many pow wow dancers use the eagle claw as part of their regalia as well. Eagle feathers are often used in traditional ceremonies, particularly in the construction of regalia worn and as a part of fans, bustles and head dresses. In the Navajo tradition an eagle feather is represented to be a protector, along with the feather Navajo medicine men use the leg and wing bones for ceremonial whistles. The Lakota, for instance, give an eagle feather as a symbol of honor to person who achieves a task. In modern times, it may be given on an event such as a graduation from college. The Pawnee consider eagles as symbols of fertility because their nests are built high off the ground and because they fiercely protect their young. The Choctaw consider the bald eagle, who has direct contact with the upper world of the sun, as a symbol of peace.
During the Sun Dance, which is practiced by many Plains Indian tribes, the eagle is represented in several ways. The eagle nest is represented by the fork of the lodge where the dance is held. A whistle made from the wing bone of an eagle is used during the course of the dance. Also during the dance, a medicine man may direct his fan, which is made of eagle feathers, to people who seek to be healed. The medicine man touches the fan to the center pole and then to the patient, in order to transmit power from the pole to the patient. The fan is then held up toward the sky, so that the eagle may carry the prayers for the sick to the Creator.
Current eagle feather law stipulates that only individuals of certifiable Native American ancestry enrolled in a federally recognized tribe are legally authorized to obtain or possess bald or golden eagle feathers for religious or spiritual use. The constitutionality of these laws has been questioned by Native American groups on the basis that it violates the First Amendment by affecting ability to practice their religion freely.
The National Eagle Repository, a division of the FWS, exists as a means to receive, process, and store bald and golden eagles which are found dead and to distribute the eagles, their parts and feathers to federally recognized Native American tribes for use in religious ceremonies.
National symbol of the United States | Bald eagle | Wikipedia | 500 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The bald eagle is the national symbol of the United States. It was adopted as a national emblem in 1782, but not designated the "national bird" until an act of Congress in December 2024.
The founders of the United States were fond of comparing their new republic with the Roman Republic, in which eagle imagery (usually involving the golden eagle) was prominent. On June 20, 1782, the Continental Congress adopted the design for the Great Seal of the United States, depicting a bald eagle grasping 13 arrows and an olive branch with thirteen leaves with its talons.
The bald eagle appears on most official seals of the U.S. government, including the presidential seal, the presidential flag, and in the logos of many U.S. federal agencies. Between 1916 and 1945, the presidential flag (but not the seal) showed an eagle facing to its left (the viewer's right), which gave rise to the urban legend that the flag is changed to have the eagle face towards the olive branch in peace, and towards the arrows in wartime.
Contrary to popular legend, there is no evidence that Benjamin Franklin ever publicly supported the wild turkey (Meleagris gallopavo), rather than the bald eagle, as a symbol of the United States. However, in a letter written to his daughter in 1784 from Paris, criticizing the Society of the Cincinnati, he stated his personal distaste for the bald eagle's behavior. In the letter Franklin states:
Franklin opposed the creation of the Society because he viewed it, with its hereditary membership, as a noble order unwelcome in the newly independent Republic, contrary to the ideals of Lucius Quinctius Cincinnatus, for whom the Society was named. His reference to the two kinds of birds is interpreted as a satirical comparison between the Society of the Cincinnati and Cincinnatus.
Popular culture
Largely because of its role as a symbol of the United States, but also because of its being a large predator, the bald eagle has many representations in popular culture. In film and television depictions the call of the red-tailed hawk, which is much louder and more powerful, is often substituted for bald eagles. | Bald eagle | Wikipedia | 436 | 4401 | https://en.wikipedia.org/wiki/Bald%20eagle | Biology and health sciences | Accipitrimorphae | Animals |
The brown bear (Ursus arctos) is a large bear native to Eurasia and North America. Of the land carnivorans, it is rivaled in size only by its closest relative, the polar bear, which is much less variable in size and slightly bigger on average. The brown bear is a sexually dimorphic species, as adult males are larger and more compactly built than females. The fur ranges in color from cream to reddish to dark brown. It has evolved large hump muscles, unique among bears, and paws up to wide and long, to effectively dig through dirt. Its teeth are similar to those of other bears and reflect its dietary plasticity.
Throughout the brown bear's range, it inhabits mainly forested habitats in elevations of up to . It is omnivorous, and consumes a variety of plant and animal species. Contrary to popular belief, the brown bear derives 90% of its diet from plants. When hunting, it will target animals as small as insects and rodents to those as large as moose or muskoxen. In parts of coastal Alaska, brown bears predominantly feed on spawning salmon that come near shore to lay their eggs. For most of the year, it is a usually solitary animal that associates only when mating or raising cubs. Females give birth to an average of one to three cubs that remain with their mother for 1.5 to 4.5 years. It is a long-lived animal, with an average lifespan of 25 years in the wild. Relative to its body size, the brown bear has an exceptionally large brain. This large brain allows for high cognitive abilities, such as tool use. Attacks on humans, though widely reported, are generally rare. | Brown bear | Wikipedia | 344 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
While the brown bear's range has shrunk, and it has faced local extinctions across its wide range, it remains listed as a least concern species by the International Union for Conservation of Nature (IUCN) with a total estimated population in 2017 of 110,000. Populations that were hunted to extinction in the 19th and 20th centuries are the Atlas bear of North Africa and the Californian, Ungavan and Mexican populations of the grizzly bear of North America. Many of the populations in the southern parts of Eurasia are highly endangered as well. One of the smaller-bodied forms, the Himalayan brown bear, is critically endangered: it occupies only 2% of its former range and is threatened by uncontrolled poaching for its body parts. The Marsican brown bear of central Italy is one of several currently isolated populations of the Eurasian brown bear and is believed to have a population of only about 50 bears.
The brown bear is considered to be one of the most popular of the world's charismatic megafauna. It has been kept in zoos since ancient times, and has been tamed and trained to perform in circuses and other acts. For thousands of years, the brown bear has had a role in human culture, and is often featured in literature, art, folklore, and mythology.
Etymology
The brown bear is sometimes referred to as the , from Middle English. This name originated in the fable History of Reynard the Fox, translated by William Caxton, from the Middle Dutch word or , meaning "brown". In the mid-19th-century United States, the brown bear was given the nicknames "Old Ephraim" and "Moccasin Joe".
The scientific name of the brown bear, Ursus arctos, comes from the Latin , meaning "bear", and the Greek /, also meaning "bear".
Evolution and taxonomy
Taxonomy and subspecies
Carl Linnaeus scientifically described the species under the name Ursus arctos in the 1758 edition of Systema Naturae. Brown bear taxonomy and subspecies classification has been described as "formidable and confusing", with few authorities listing the same set of subspecies. There are hundreds of obsolete brown-bear subspecies. As many as 90 subspecies have been proposed. A 2008 DNA analysis identified as few as five main clades, which comprise all extant brown bear species, while a 2017 phylogenetic study revealed nine clades, including one representing polar bears. , 15 extant, or recently extinct, subspecies were recognized by the general scientific community. | Brown bear | Wikipedia | 508 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
DNA analysis shows that, apart from recent, human-caused population fragmentation, brown bears in North America are generally part of a single interconnected population system, with the exception of the population (or subspecies) in the Kodiak Archipelago, which has probably been isolated since the end of the last Ice Age. These data demonstrate that U. a. gyas, U. a. horribilis, U. a. sitkensis, and U. a. stikeenensis are not distinct or cohesive groups, and would more accurately be described as ecotypes. For example, brown bears in any particular region of the Alaska coast are more closely related to adjacent grizzly bears than to distant populations of brown bears.
The history of the bears of the Alexander Archipelago is unusual in that these island populations carry polar bear DNA, presumably originating from a population of polar bears that was left behind at the end of the Pleistocene, but have since been connected with adjacent mainland populations through the movement of males, to the point where their nuclear genomes indicate more than 90% brown bear ancestry. MtDNA analysis revealed that brown bears are apparently divided into five different clades, some of which coexist or co-occur in different regions.
Evolution
The brown bear is one of eight extant species in the bear family Ursidae and of six extant species in the subfamily Ursinae.
The brown bear is thought to have evolved from the Etruscan bear (Ursus etruscus) in Asia during the early Pliocene. A genetic analysis indicated that the brown bear lineage diverged from the cave bear species-complex approximately 1.2–1.4 million years ago, but did not clarify if U. savini persisted as a paraspecies for the brown bear before perishing. The oldest brown bear fossils occur in Asia from about 500,000 to 300,000 years ago. They entered Europe 250,000 years ago and North Africa shortly after. Brown bear remains from the Pleistocene period are common in the British Isles, where, amongst other factors, they may have contributed to the extinction of cave bears (Ursus spelaeus). | Brown bear | Wikipedia | 435 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears first emigrated to North America from Eurasia via Beringia during the Illinoian Glaciation. Genetic evidence suggests that several brown bear populations migrated into North America, aligning with the glacial cycles of the Pleistocene. The founding population of most North American brown bears arrived first, with the genetic lineage developing around ~177,000 BP. Genetic divergences suggest that brown bears first migrated south during MIS-5 (~92,000–83,000 BP), upon the opening of the ice-free corridor, After a local extinction in Beringia ~33,000 BP, two new but closely related lineages repopulated Alaska and northern Canada from Eurasia after the Last Glacial Maximum (>25,000 BP).
Brown-bear fossils discovered in Ontario, Ohio, Kentucky, and Labrador show that the species occurred farther east than indicated in historic records. In North America, two types of the subspecies Ursus arctos horribilis are generally recognized—the coastal brown bear and the inland grizzly bear.
Hybrids
A grizzly–polar bear hybrid is a rare ursid hybrid resulting from a crossbreeding of a brown bear with a polar bear. It has occurred both in captivity and in the wild. In 2006, the occurrence of this hybrid was confirmed by testing the DNA of a strange-looking bear that had been shot in the Canadian Arctic, and seven more hybrids have since been confirmed in the same region, all descended from a single female polar bear. Previously, the hybrid had been produced in zoos and was considered a "cryptid" (a hypothesized animal for which there is no scientific proof of existence in the wild). Analyses of the genomes of bears have shown that introgression between species was widespread during the evolution of the genus Ursus, including the introgression of polar-bear DNA introduced to brown bears during the Pleistocene.
Description
Size
The brown bear is the most variable in size of modern bears. The typical size depends upon which population it is from, as most accepted subtypes vary widely in size. This is in part due to sexual dimorphism, as male brown bears average at least 30% larger than females in most subtypes. Individual bears vary in size seasonally, weighing the least in spring due to lack of foraging during hibernation, and the most in late fall, after a period of hyperphagia to put on additional weight to prepare for hibernation. | Brown bear | Wikipedia | 501 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears generally weigh , with males outweighing females. They have a head-and-body length of and a shoulder height of . The tail is relatively short, as in all bears, ranging from in length. The smallest brown bears, females during spring among barren-ground populations, can weigh so little as to roughly match the body mass of males of the smallest living bear species, the sun bear (Helarctos malayanus), while the largest coastal populations attain sizes broadly similar to those of the largest living bear species, the polar bear. Brown bears of the interior are generally smaller, being around the same weight as an average lion, at an average of in males and in females, whereas adults of the coastal populations weigh about twice as much. The average weight of adult male bears, from 19 populations, was found to be while adult females from 24 populations were found to average .
Coloration
Brown bears are often not fully brown. They have long, thick fur, with a moderately elongated mane at the back of the neck which varies somewhat across bear types. In India, brown bears can be reddish with silver-tipped hairs, while in China, brown bears are bicolored, with a yellowish-brown or whitish collar across the neck, chest, and shoulders. Even within well-defined subspecies, individuals may show highly variable hues of brown. North American grizzlies can be from dark brown (almost black) to cream (almost white) or yellowish-brown and often have darker-colored legs. The common name "grizzly" stems from their typical coloration, with the hairs on their back usually being brownish-black at the base and whitish-cream at the tips, giving them their distinctive "grizzled" color. Apart from the cinnamon subspecies of the American black bear (U. americanus cinnamonum), the brown bear is the only modern bear species to typically appear truly brown. The brown bear's winter fur is very thick and long, especially in northern subspecies, and can reach at the withers. The winter hairs are thin, yet rough to the touch. The summer fur is much shorter and sparser, with its length and density varying among geographic ranges.
Cranial morphology and size | Brown bear | Wikipedia | 450 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Adults have massive, heavily built, concave skulls, which are large in proportion to the body. The projections of the skull are well developed. Skull lengths of Russian brown bears tend to be for males, and for females. Brown bears have the broadest skull of any extant ursine bear. The width of the zygomatic arches in males is , and in females. Brown bears have strong jaws: the incisors and canine teeth are large, with the lower canines being strongly curved. The first three molars of the upper jaw are underdeveloped and single-crowned with one root. The second upper molar is smaller than the others, and is usually absent in adults. It is usually lost at an early age, leaving no trace of its alveolus in the jaw. The first three molars of the lower jaw are very weak, and are often lost at an early age. The teeth of brown bears reflect their dietary plasticity and are broadly similar to those of other bears. They are reliably larger than teeth of American black bears, but average smaller in molar length than those of polar bears.
Claws and feet
Brown bears have large, curved claws, with the front ones being larger than the back. They may reach and measure along the curve. Compared with the American black bear (Ursus americanus), the brown bear has longer and stronger claws, with a blunt curve. Due to their claw structure, in addition to their excessive weight, adult brown bears are not able to climb trees as well as black bears. In rare cases adult female brown bears have been seen scaling trees. The claws of a polar bear are quite different, being notably shorter but broader with a strong curve and sharper point. The species has large paws; the rear feet measure long, while the forefeet tend to measure 40% less. Brown bears are the only extant bears with a hump at the top of their shoulder, which is made entirely of muscle. This feature developed presumably to impart more force in digging, which helps during foraging and facilitates den construction prior to hibernation.
Distribution and habitat | Brown bear | Wikipedia | 432 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears inhabit the broadest range of habitats of any living bear species. They seem to have no altitudinal preferences and have been recorded from sea level to an elevation of in the Himalayas. In most of their range, brown bears seem to prefer semi-open country, with a scattering of vegetation, that can allow them a resting spot during the day. However, they have been recorded as inhabiting every variety of northern temperate forest known to occur.
This species was once native to Europe, much of Asia, the Atlas Mountains of Africa, and North America, but are now extirpated in some areas, and their populations have greatly decreased in other areas. There are approximately 200,000 brown bears left in the world. The largest populations are in Russia with 130,000, the United States with 32,500, and Canada with around 25,000. Brown bears live in Alaska, east through the Yukon and Northwest Territories, south through British Columbia, and through the western half of Alberta. The Alaskan population is estimated at a healthy 30,000 individuals. In the lower 48 states, they are repopulating slowly, but steadily along the Rockies and the western Great Plains. | Brown bear | Wikipedia | 240 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
In Europe, in 2010, there were 14,000 brown bears in ten fragmented populations, from Spain (estimated at only 20–25 animals in the Pyrenees in 2010, in a range shared between Spain, France, and Andorra, and some 210 animals in Asturias, Cantabria, Galicia, and León, in the Picos de Europa and adjacent areas in 2013) in the west, to Russia in the east, and from Sweden and Finland in the north to Romania (5,000–6,000), Bulgaria (900–1,200), Slovakia (with about 600–800 animals), Slovenia (500–700 animals), and Greece (with Karamanlidis et al. 2015 estimating >450 animals) in the south. In Asia, brown bears are found primarily throughout Russia, thence more spottily southwest to parts of the Middle East, including the Eastern Black Sea Region, Turkey which has 5,432 individuals of brown bear, to as far south as southwestern Iran, and to the southeast in Northeast China. Brown bears are also found in Western China, Kyrgyzstan, North Korea, Pakistan, Afghanistan, and India. A population of brown bears can be found on the Japanese island of Hokkaidō, which holds the largest number of non-Russian brown bears in eastern Asia, with about 2,000–3,000 animals.
Conservation status
While the brown bear's range has shrunk and it has faced local extinctions, it remains listed as a least-concern species by the IUCN, with a total population of approximately 200,000. , the brown bear and the American black bear are the only bear species not classified as threatened by the IUCN. However, the California grizzly bear, Ungava brown bear, Atlas bear, and Mexican grizzly bear, as well as brown bear populations in the Pacific Northwest, were hunted to extinction in the 19th and early 20th centuries and many of the southern Asian subspecies are highly endangered. The Syrian brown bear (U. a. syriacus) is very rare and it has been extirpated from more than half of its historic range. One of the smallest-bodied subspecies, the Himalayan brown bear (U. a. isabellinus), is critically endangered: it occupies only 2% of its former range and is threatened by uncontrolled poaching for its body parts. The Marsican brown bear in central Italy is believed to have a population of just 50 bears. | Brown bear | Wikipedia | 500 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
The smallest populations are most vulnerable to habitat loss and fragmentation, whereas the largest are primarily threatened by overhunting. The use of land for agriculture may negatively effect brown bears. Additionally, roads and railway tracks could pose a serious threat, as oncoming vehicles may collide with crossing animals. Poaching has been cited as another mortality factor. In one instance, a 3-year-long survey in the Russian Far East detected the illegal shipping of brown bear gallbladders to Southeast Asian countries. The purpose and motive behind the trade is unknown.
An action plan in 2000 aimed to conserve brown bears in Europe by mitigating human–wildlife conflict, educating farm owners as to sustainable practices, and preserving and expanding remaining forests. Compensation was given to people who suffered losses of livestock, food supplies, or shelter. Growing bear populations were recorded in some countries, such as Sweden, where an increase of 1.5% per annum occurred between the 1940s and 1990s. Brown bears in Central Asia are primarily threatened by climate change. In response to this, conservationists plan on building wildlife corridors to promote easy access from one brown bear population to another. In Himalayan Nepal, farmers may kill brown bears in revenge for livestock predation.
Behavior and life history
A 2014 study revealed that brown bears peaked in activity around the morning and early evening hours. Although activity can happen day or night, bears that live in locations where they are apt to interact with humans are more likely to be fully nocturnal.
In areas with little interaction, many adult bears are primarily crepuscular, while yearlings and newly independent bears appear to be most active throughout the day. From summer through autumn, a brown bear can double its weight from what it was in the spring, gaining up to of fat, on which it relies to make it through winter, when it becomes lethargic. Although they are not full hibernators and can be woken easily, both sexes prefer to den in a protected spot during the winter months. Hibernation dens may be located at any spot that provides cover from the elements and that can accommodate their bodies, such as a cave, crevice, cavernous tree roots, or hollow logs. | Brown bear | Wikipedia | 443 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears have one of the largest brains of any extant carnivoran relative to their body size and have been shown to engage in tool use, which requires advanced cognitive abilities. This species is mostly solitary, although bears may gather in large numbers at major food sources (e.g., open garbage dumps or rivers containing spawning salmon) and form social hierarchies based on age and size. Adult male bears are particularly aggressive and are avoided by adolescent and subadult males, both at concentrated feeding opportunities and chance encounters. Females with cubs rival adult males in aggression and are much more intolerant of other bears than single females. Young adolescent males tend to be least aggressive and have been observed in nonantagonistic interactions with each other. Dominance between bears is asserted by making a frontal orientation, showing off canine teeth, muzzle twisting, and neck stretching, to which a subordinate will respond with a lateral orientation, by turning away and dropping the head, and by sitting or lying down. During combat, bears use their paws to strike their opponents in the chest or shoulders and bite the head or neck.
Communication
Several different facial expressions have been documented in brown bears. The "relaxed-face" is made during everyday activities, a face where the ears pointed to the sides and the mouth closed or slackly open. During social play, bears make "relaxed open-mouth face" in which the mouth is open, with a curled upper lip and hanging lower lip, and the ears alert and shifting. When looking at another animal at a distance, the bear makes an "alert face" as the ears are cocked and alert, the eyes wide open with the mouth is closed or only open slightly. The "tense closed mouth face" is made with the ears laid back and the mouth closed, and occurs when the bear feels threatened. When approached by another individual, the animal makes a "puckered-lip face" with a protruding upper lip and ears that go from cocked and alert when at a certain distance to laid back when closer or when retreating. The "jaw gape face" consists of an open mouth with visible lower canines and hanging lips while the "biting face" is similar to the "relaxed open-mouth face" except the ears are flattened and the eyes are wide enough to expose the sclera. Both the "jaw gape face" and the "biting face" are made when the bear is aggressive and can quickly switch between them. | Brown bear | Wikipedia | 496 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears also produce various vocalizations. Huffing occurs when the animal is tense, while woofing is made when alarmed. Both sounds are produced by exhalations, though huffing is harsher and is made continuously (approximately twice per second). Growls and roars are made when aggressive. Growling is "harsh" and "guttural" and can range from a simple grrr to a rumble. A rumbling growl can escalate to a roar when the bear is charging. Roaring is described as "thunderous" and can travel . Mothers and cubs wanting physical contact will bawl, which is heard as waugh!, waugh!.
Home ranges
Brown bears usually inhabit vast home ranges; however, they are not highly territorial. Several adult bears roam freely over the same vicinity without contention, unless rights to a fertile female or food sources are being contested. Despite their lack of traditional territorial behavior, adult males seem to have a "personal zone" within which other bears are not tolerated if they are seen. Males always wander further than females, due to such behavior giving increasing access to both females and food sources. Females have the advantage of inhabiting smaller territories, which decreases the likelihood of encounters with male bears who may endanger their cubs.
In areas where food is abundant, such as coastal Alaska, home ranges for females and males are up to and , respectively. Similarly, in British Columbia, bears of the two sexes travel in relatively compact home ranges of . In Yellowstone National Park, home ranges for females are up to and up to for males. In Romania, the largest home range was recorded for adult males (). In the central Arctic of Canada, where food sources are quite scarce, home ranges range up to for females and for males.
Reproduction | Brown bear | Wikipedia | 357 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
The mating season occurs from mid-May to early July, shifting to later in the year the farther north the bears are found. Brown bears are polygynandrous, remaining with the same mate for a couple of days to a couple of weeks and mating multiply during the mating season. Outside of this narrow time frame, adult male and female brown bears show no sexual interest in each other. Females mature sexually between the ages of four and eight. Males first mate about a year later, when they are large and strong enough to compete with other males for mating rights. Males will try to mate with as many females as they can; usually a successful male will mate with two females in a span of one to three weeks. Similarly, adult female brown bears can mate with up to four, sometimes even eight, males while in oestrus (heat), potentially mating with two in a single day. Females come into oestrus every three to four years, with an outside range of 2.4 to 5.7 years. The urine markings of a female in oestrus can attract several males via scent. Dominant males may try to sequester a female for her entire oestrus period of approximately two weeks, but usually are unable to retain her for the entire time. Copulation is prolonged and lasts for over 20 minutes. | Brown bear | Wikipedia | 270 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Males take no part in raising cubs – parenting is left entirely to the females. Through the process of delayed implantation, a female's fertilized egg divides and floats freely in the uterus for six months. During winter dormancy, the fetus attaches to the uterine wall. The cubs are born eight weeks later, while the mother sleeps. If the mother does not gain enough weight to survive through the winter while gestating, the embryo does not implant and is reabsorbed into the body. Litters consist of as many as six cubs, though litters of one to three are more typical. The size of a litter depends on factors such as geographic location and food supply. At birth, cubs are blind, toothless and hairless and may weigh . There are records of females sometimes adopting stray cubs or even trading or kidnapping cubs when they emerge from hibernation (a larger female may claim cubs from a smaller one). Older and larger females within a population tend to give birth to larger litters. The cubs feed on their mother's milk until spring or early summer, depending on climate conditions. At this time, the cubs weigh and have developed enough to follow and forage for solid food with their mother over long distances. | Brown bear | Wikipedia | 256 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
The cubs are dependent on the mother and a close bond is formed. During the dependency stage, the cubs learn (rather than inherit as instincts from birth) survival techniques, such as which foods have the highest nutritional value and where to obtain them; how to hunt, fish, and defend themselves; and where to den. Increased brain size in large carnivores has been positively linked to whether a given species is solitary, as is the brown bear, or raises offspring communally. Thus, the relatively large, well-developed brain of a female brown bear is presumably key in teaching behavior. The cubs learn by following and imitating their mother's actions during the period they are with her. Cubs remain with their mother for an average of 2.5 years in North America, and gain independence from as early as 1.5 years of age to as late as 4.5 years. The stage at which independence is attained may generally be earlier in some parts of Eurasia, as the latest date which mother and cubs were together was 2.3 years. Most families separated in under two years in a study in Hokkaido, and in Sweden most yearlings were their own. Brown bears practice infanticide, as an adult male bear may kill the cubs of another. When an adult male brown bear kills a cub, it is usually because he is trying to bring the female into oestrus, as she will enter that state within two to four days after the death of her cubs. Cubs may flee up a tree when they see a strange male bear approaching. The mother often successfully defends them, even though the male may be twice as heavy as she. However, females have been known to die in such confrontations.
Dietary habits | Brown bear | Wikipedia | 348 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
The brown bear is one of the most omnivorous animals and has been recorded as consuming the greatest variety of foods of any bear. Despite their reputation, most brown bears are not highly carnivorous, as they derive up to 90% of their dietary food energy from vegetable matter. They often feed on a variety of plant life, including berries, grasses, flowers, acorns, and pine cones, as well as fungi such as mushrooms. Among all bears, brown bears are uniquely equipped to dig for tough foods such as roots, bulbs, and shoots. They use their long, strong claws to dig out earth to reach roots and their powerful jaws to bite through them. In spring, winter-provided carrion, grasses, shoots, sedges, moss, and forbs are the dietary mainstays for brown bears internationally. Fruits, including berries, become increasingly important during summer and early autumn. Roots and bulbs become critical in autumn for some inland bear populations if fruit crops are poor.
They will also commonly consume animal matter, which in summer and autumn may regularly be in the form of insects, larvae, and grubs, including beehives. Bears in Yellowstone eat an enormous number of moths during the summer, sometimes as many as 40,000 army cutworm moths in a single day, and may derive up to half of their annual food energy from these insects. Brown bears living near coastal regions will regularly eat crabs and clams. In Alaska, bears along the beaches of estuaries regularly dig through the sand for clams. This species may eat birds and their eggs, including almost entirely ground- or rock-nesting species. The diet may be supplemented by rodents or similar small mammals, including marmots, ground squirrels, mice, rats, lemmings, and voles. With particular regularity, bears in Denali National Park will wait at burrows of Arctic ground squirrels, hoping to pick off a few of those rodents. | Brown bear | Wikipedia | 392 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
In the Kamchatka peninsula and several parts of coastal Alaska, brown bears feed mostly on spawning salmon, whose nutrition and abundance explain the enormous size of the bears in those areas. The fishing techniques of bears are well-documented. They often congregate around falls when the salmon are forced to breach the water, at which point the bears will try to catch the fish in mid-air (often with their mouths). They will also wade into shallow water, hoping to pin a slippery salmon with their claws. While they may eat almost all the parts of the fish, bears at the peak of salmon spawning, when there is usually a glut of fish to feed on, may eat only the most nutrious parts of the salmon (including the eggs and head) and then indifferently leave the rest of the carcass to scavengers, which can include red foxes, bald eagles, common ravens, and gulls. Despite their normally solitary habits, brown bears will gather closely in numbers at good spawning sites. The largest and most powerful males claim the most fruitful fishing spots and will sometimes fight over the rights to them.
Beyond the regular predation of salmon, most brown bears are not particularly active predators. While perhaps a majority of bears of the species will charge at large prey at one point in their lives, many predation attempts start with the bear clumsily and half-heartedly pursuing the prey and end with the prey escaping alive. On the other hand, some brown bears are quite self-assured predators who habitually pursue and catch large prey. Such bears are usually taught how to hunt by their mothers from an early age. Large mammals preyed on can include various ungulate species such as elk, moose, caribou, muskoxen, and wild boar. When brown bears attack these large animals, they usually target young or infirm ones, which are easier to catch. Typically when hunting (especially young prey), the bear pins its prey to the ground and then immediately tears at and eats it alive. It will also bite or swipe some prey to stun it enough to knock it over for consumption. In general, large mammalian prey is killed with raw strength and bears do not display the specialized killing methods of felids and canids. To pick out young or infirm individuals, bears will charge at herds so the more vulnerable, and thus slower-moving, individuals will become apparent. Brown bears may ambush young animals by finding them via scent. | Brown bear | Wikipedia | 508 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
When emerging from hibernation, brown bears, whose broad paws allow them to walk over most ice and snow, may pursue large prey such as moose, whose hooves cannot support them on encrusted snow. Similarly, predatory attacks on large prey sometimes occur at riverbeds, when it is more difficult for the prey specimen to run away due to muddy or slippery soil. On rare occasions, while confronting fully-grown, dangerous prey, bears kill them by hitting with their powerful forearms, which can break the necks and backs of large creatures such as adult moose and adult bison. They feed on carrion, and use their size to intimidate other predators – such as wolves, cougars, tigers, and American black bears – away from their kills. Carrion is especially important in the early spring (when the bears are emerging from hibernation), much of it comprising winter-killed big game. Cannibalism is not unheard of, though predation is not normally believed to be the primary motivation when brown bears attack each other.
When forced to live in close proximity with humans and their domesticated animals, bears may potentially predate any type of domestic animal. Among these, domestic cattle are sometimes exploited as prey. Cattle are bitten on the neck, back, or head, and then the abdominal cavity is opened for eating. Plants and fruit farmed by humans are readily consumed as well, including corn, wheat, sorghum, melons, and any form of berries. They may feed on domestic bee yards, readily consuming both honey and the brood (grubs and pupae) of the honey bee colony. Human foods and trash are eaten when possible. When an open garbage dump was kept in Yellowstone, brown bears were one of the most voracious and regular scavengers. The dump was closed after both brown and American black bears came to associate humans with food and lost their natural fear of them.
Relations with other predators | Brown bear | Wikipedia | 396 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Adult bears are generally immune to predatory attacks except from large Siberian (Amur) tigers and other bears. Following a decrease of ungulate populations from 1944 to 1959, 32 cases of Siberian tigers attacking both Ussuri brown bears (Ursus arctos lasiotus) and Ussuri black bears (U. thibetanus ussuricus) were recorded in the Russian Far East, and bear hairs were found in several tiger scat samples. Tigers attack black bears less often than brown bears, since the brown bears live in more open habitats and are not able to climb trees. In the same time period, four cases of brown bears killing female tigers and young cubs were reported, both in disputes over prey and in self-defense. In rare cases, when Amur tigers prey on brown bears, they usually target young and sub-adult bears, besides small female adults taken outside their dens, generally when lethargic from hibernation. Predation by tigers on denned brown bears was not detected during a study carried out between 1993 and 2002. Ussuri brown bears, along with the smaller black bears constitute 2.1% of the Siberian tiger's annual diet, of which 1.4% are brown bears.
Brown bears regularly intimidate wolves to drive them away from their kills. In Yellowstone National Park, bears pirate wolf kills so often, Yellowstone's Wolf Project director Doug Smith wrote, "It's not a matter of if the bears will come calling after a kill, but when." Despite the animosity between the two species, most confrontations at kill sites or large carcasses end without bloodshed on either side. Though conflict over carcasses is common, on rare occasions the two predators tolerate each other at the same kill. To date, there is a single recorded case of fully-grown wolves being killed by a grizzly bear. Given the opportunity, however, both species will prey on the other's cubs. In some areas, grizzly bears regularly displace cougars from their kills. Cougars kill small bear cubs on rare occasions, but there was only one report of a bear killing a cougar, of unknown age and condition, between 1993 and 1996. | Brown bear | Wikipedia | 449 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears usually dominate other bear species in areas where they coexist. Due to their smaller size, American black bears are at a competitive disadvantage to grizzly bears in open, unforested areas. Although displacement of black bears by grizzly bears has been documented, actual killing of black bears by grizzlies has only occasionally been reported. Confrontation is mostly avoided due to the black bear's diurnal habits and preference for heavily forested areas, as opposed to the grizzly's largely nocturnal habits and preference for open spaces. Brown bears may also kill Asian black bears, though the latter species probably largely avoids conflicts with the brown bear, due to similar habits and habitat preferences to the American black species. As of the 21st century, there has been an increase in interactions between brown bears and polar bears, theorized to be caused by climate change. Brown and grizzly bears have been seen moving increasingly northward into territories formerly claimed by polar bears. They tend to dominate polar bears in disputes over carcasses, and dead polar bear cubs have been found in brown bear dens.
Longevity and mortality
The brown bear has a naturally long life. Wild females have been observed reproducing at 28 years, which is the oldest known age for reproduction of any ursid in the wild. The peak reproductive age for females ranges from four to 20 years old. The lifespan of both sexes within minimally hunted populations is estimated at an average of 25 years. The oldest recorded wild individual was nearly 37 years old. In captivity, the oldest recorded female was around 40 years old, while males have been known to live up to 47 years.
While male bears potentially live longer in captivity, female grizzly bears have a greater annual survival rate than males within wild populations, per a study done in the Greater Yellowstone Ecosystem. Annual mortality for bears of any age is estimated at 10% in most protected areas. Around 13% to 44% of cubs die within their first year. Beyond predation by large predators – including wolves, Siberian tigers, and other brown bears – starvation and accidents also claim the lives of cubs. Studies have indicated that the most prevalent cause of mortality for first-year cubs is malnutrition. | Brown bear | Wikipedia | 445 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brown bears are susceptible to parasites such as flukes, ticks, tapeworms, roundworms, and biting lice. It is thought that brown bears may catch canine distemper virus (CDV) from other caniforms such as stray dogs and wolves. A captive individual allegedly succumbed to Aujeszky's disease.
Hibernation physiology
A study conducted by the Brown Bear Research Project did a proteomic analysis of the brown bear's blood, organs, and tissues to pinpoint proteins and peptides that either increased or decreased in expression in the winter and summer months. One major finding was that the presence of the plasma protein sex hormone-binding globulin (SHBG) increased by 45 times during the brown bear's hibernation period. Although scientists do not yet understand the role of SHBG in maintaining the brown bear's health, some believe these findings could potentially help in understanding and preventing human diseases that come from a sedentary lifestyle.
Relations with humans
Attacks on humans
Brown bears usually avoid areas where extensive development or urbanization has occurred. They usually avoid people and rarely attack on sight. They are, however, unpredictable in temperament, and may attack if threatened or surprised. Mothers defending cubs are the most prone to attacking, being responsible for 70% of brown bear-caused human fatalities in North America. Attacks tend to result in serious injury and, in some cases, death. Due to the bears' enormous physical strength, a single bite or swipe can be deadly. Violent encounters with brown bears usually last a few minutes, though they can be prolonged if the victims fight back.
A study conducted in 2019 found that 664 bear attacks were reported during a 15-year period (20002015) throughout North America and Eurasia. There were 568 injuries and 95 fatalities. Around 10 people a year are killed by brown bears in Russia, more than all the other parts of the brown bear's range combined. In Japan, a large brown bear nicknamed Kesagake ("kesa-style slasher") caused the worst brown bear attack in Japanese history in Tomamae, Hokkaidō, during numerous encounters during December 1915. It killed seven people and wounded three others before being gunned down during a large-scale beast-hunt. A study by U.S. and Canadian researchers has found bear spray to be more effective at stopping aggressive bear behavior than guns, working in 92% of studied incidents, versus 67% for guns.
Bear hunting | Brown bear | Wikipedia | 511 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Humans have been recorded hunting brown bears for over 10,300–9,300 years. Bears were hunted throughout their range in Europe, Asia and North America by both the native Americans and Europeans. The former usually killed bears for survival needs, while the latter for sports or population control. In Europe, between the 17th and 18th centuries, humans sought to control brown bear numbers by awarding those who managed to kill one. This bounty scheme pushed the brown bear population to the brink of extinction before comprehensive protection was offered in the 1900s. Despite this, a 2018 study found hunting to be one of the contributing factors to the drop in brown bear numbers in northern Europe.
The earliest known case of a European killing a grizzly bear dates back to 1691. Their arrival in western United States led to the extirpation of local brown bear populations in the 19th and early 20th centuries. During the early years of European settlement in North America, bears were usually killed with a spear or lasso rope. The introduction of rifles in the mid-19th century largely facilitated bear hunts, which allowed for an increasing trend. Bears were also pitted into fights against male cattle, often ending with either animal grievously injured or dead. The last two decades of the 19th century saw an increase in bounties. Conflicts with farmers also contributed to its rapid decline. It wasn't until the 1920s that grizzly bears received some type of protection from the US government. Today, brown bears are legally hunted in some American states, such as Alaska. However, a hunting license is required and killings of nurturing females and cubs will result in a prison sentence.
Brown bear meat is sometimes consumed and used in recipes such as dumplings, hams and stews. The Indigenous people of James Bay (Eastern) Cree use their flesh in traditional dishes. In Asia and Romania, the paws are consumed as exotic delectables; they have been a prevalent component of traditional Chinese food since 500 BC. The total weight of commercially sold brown bear meat is estimated at 17 tons annually.
In captivity | Brown bear | Wikipedia | 414 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Bears have been recorded in captivity as early as 1,500 BC. As of 2017, there are more than 700 brown bears in zoos and wildlife parks worldwide. Captive bears are largely lethargic and spend a considerable amount of time doing nothing. When active, captive bears may engage in repetitive back and forth motion, known widely as pacing. This behavior is most prevalent in bears kept in small, cramped cages often with no natural setting. Pacing is a way of coping with stress that comes with being trapped in unnaturally small spaces. These stereotyped behaviors have decreased due to better and larger enclosures being built, and more sustainable management from zoo staff.
Starting from infancy, brown bears may also be exploited as dancing bears. Cubs, for example, are positioned on hot metal plates, causing them to "dance" to the sound of violin music running in the background. The process is repeated, resulting in bears being trained to "dance" when a violin is played. Similarly, brown bears are displayed in tiny enclosures near a restaurant, mainly for the purpose of luring customers. Privately-owned bears are also placed in insufficient environments and often suffer from malnutrition and obesity.
Brown bears have been popular attractions at circuses and other acts since ancient times. Due to their large size and imposing demeanor, the Romans used brown bears in the execution of criminals, and pitted bears in fights with other animals. Gladiators would also fight bears, in what was essentially a fight to the death. Such events occurred in amphitheaters housing thousands of spectators. In later times, street performances became popular in the Middle Ages; acts included "dancing" and "sleeping on command". These performances became increasingly widespread, and from the 1700s to 1800s, traveling circuses would perform in the streets of many European and Asian countries. Such circuses made use of bears that wore special clothing, and were usually run by musicians. A short while later, modern circuses began utilizing bears around the second half of the 18th century. Brown bears were said to be the easiest bear species to train due to their intelligence, unique personalities, and exceptional stability. According to a 2009 analysis, the brown bear was the second most exploited circus animal after the tiger.
Culture | Brown bear | Wikipedia | 453 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Bears have been popular subjects in art, literature, folklore, and mythology. The image of the mother bear was prevalent throughout societies in North America and Eurasia, based on the female's devotion and protection of her cubs. The earliest cave paintings of bears occurred in the Paleolithic, with over 100 recorded paintings. Brown bears often figure in the literature of Europe and North America as "cute and cuddly", in particular that which is written for children. "The Brown Bear of Norway" is a Scottish fairy tale telling of the adventures of a girl who married a prince magically turned into a bear and who managed to get him back into a human form by the force of her love after many trials and tribulations. With "Goldilocks and the Three Bears", a story from England, the Three Bears are usually depicted as brown bears. In German-speaking countries, children are often told the fairytale of "Snow White and Rose Red"; the handsome prince in this tale has been transfigured into a brown bear. In the United States, parents often read their preschool age children the book Brown Bear, Brown Bear, What Do You See? to teach them their colors and how they are associated with different animals. Smokey Bear, the famous mascot of U.S. Forest Service, has since the 1940s been used to educate people on the dangers of human-caused wildfire.
Brown bears have been extensively featured in the culture of Native Americans, and are considered sacred. To stop a bear's spirit from escaping after it was killed, the Denaa people severed all 4 of its paws. They delayed consuming brown-bear flesh, owing to the belief that the bear's spirit was overwhelming in fresh kills. In addition, community members that wore bear claw necklaces were highly respected, as wearing one was seen as a sign of bravery and honor. The clattering caused by repeatedly shaking these necklaces were believed to bring forth therapeutic powers. In Haida culture, one legend has it that a marriage between a woman and a grizzly bear commenced the lineage of the native people. This is thought to have allowed the Haida to thrive in bear country. | Brown bear | Wikipedia | 443 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
There is evidence of prehistoric bear worship, though this is disputed by archaeologists. It is possible that bear worship existed in early Chinese and Ainu cultures. The Romans built small carved figures of bears that were used during the burials of infants. In Ancient Greek mythology, bears were considered similar to humans, mainly due to their ability to stand upright. In many western stories and older fables the portrayed attributes of bears are sluggishness, foolishness, and gullibility, which contradicts the actual behavior of the species. For example, bears have been reported tricking hunters by backtracking in the snow.
In North America, the brown bear is considered a charismatic megafauna and has long piqued people's interest. The death of Bear 148 at the hands of a trophy hunter in 2017 sparked media outrage and the continued disapproval of trophy hunting. The Russian bear is a common national personification for Russia (as well as the former Soviet Union), despite the country having no officially-designated national animal. The brown bear is Finland's national animal. The grizzly bear is the state animal of Montana. The California golden bear is the state animal of California, despite being extinct. The coat of arms of Madrid depicts a bear reaching up into a madroño or strawberry tree (Arbutus unedo) to eat some of its fruit. The Swiss city of Bern's coat of arms depicts a bear and the city's name is popularly thought to derive from the German word for bear. The brown bear is depicted on the reverse of the Croatian 5-kuna coin, minted since 1993. | Brown bear | Wikipedia | 328 | 4402 | https://en.wikipedia.org/wiki/Brown%20bear | Biology and health sciences | Carnivora | null |
Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas).
This motion pattern typically consists of random fluctuations in a particle's position inside a fluid sub-domain, followed by a relocation to another sub-domain. Each relocation is followed by more fluctuations within the new closed volume. This pattern describes a fluid at thermal equilibrium, defined by a given temperature. Within such a fluid, there exists no preferential direction of flow (as in transport phenomena). More specifically, the fluid's overall linear and angular momenta remain null over time. The kinetic energies of the molecular Brownian motions, together with those of molecular rotations and vibrations, sum up to the caloric component of a fluid's internal energy (the equipartition theorem).
This motion is named after the Scottish botanist Robert Brown, who first described the phenomenon in 1827, while looking through a microscope at pollen of the plant Clarkia pulchella immersed in water. In 1900, the French mathematician Louis Bachelier modeled the stochastic process now called Brownian motion in his doctoral thesis, The Theory of Speculation (Théorie de la spéculation), prepared under the supervision of Henri Poincaré. Then, in 1905, theoretical physicist Albert Einstein published a paper where he modeled the motion of the pollen particles as being moved by individual water molecules, making one of his first major scientific contributions.
The direction of the force of atomic bombardment is constantly changing, and at different times the particle is hit more on one side than another, leading to the seemingly random nature of the motion. This explanation of Brownian motion served as convincing evidence that atoms and molecules exist and was further verified experimentally by Jean Perrin in 1908. Perrin was awarded the Nobel Prize in Physics in 1926 "for his work on the discontinuous structure of matter".
The many-body interactions that yield the Brownian pattern cannot be solved by a model accounting for every involved molecule. Consequently, only probabilistic models applied to molecular populations can be employed to describe it. Two such models of the statistical mechanics, due to Einstein and Smoluchowski, are presented below. Another, pure probabilistic class of models is the class of the stochastic process models. There exist sequences of both simpler and more complicated stochastic processes which converge (in the limit) to Brownian motion (see random walk and Donsker's theorem).
History | Brownian motion | Wikipedia | 497 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
The Roman philosopher-poet Lucretius' scientific poem "On the Nature of Things" () has a remarkable description of the motion of dust particles in verses 113–140 from Book II. He uses this as a proof of the existence of atoms:
Although the mingling, tumbling motion of dust particles is caused largely by air currents, the glittering, jiggling motion of small dust particles is caused chiefly by true Brownian dynamics; Lucretius "perfectly describes and explains the Brownian movement by a wrong example".
While Jan Ingenhousz described the irregular motion of coal dust particles on the surface of alcohol in 1785, the discovery of this phenomenon is often credited to the botanist Robert Brown in 1827. Brown was studying pollen grains of the plant Clarkia pulchella suspended in water under a microscope when he observed minute particles, ejected by the pollen grains, executing a jittery motion. By repeating the experiment with particles of inorganic matter he was able to rule out that the motion was life-related, although its origin was yet to be explained.
The first person to describe the mathematics behind Brownian motion was Thorvald N. Thiele in a paper on the method of least squares published in 1880. This was followed independently by Louis Bachelier in 1900 in his PhD thesis "The theory of speculation", in which he presented a stochastic analysis of the stock and option markets. The Brownian model of financial markets is often cited, but Benoit Mandelbrot rejected its applicability to stock price movements in part because these are discontinuous.
Albert Einstein (in one of his 1905 papers) and Marian Smoluchowski (1906) brought the solution of the problem to the attention of physicists, and presented it as a way to indirectly confirm the existence of atoms and molecules. Their equations describing Brownian motion were subsequently verified by the experimental work of Jean Baptiste Perrin in 1908.
The instantaneous velocity of the Brownian motion can be defined as , when , where is the momentum relaxation time.
In 2010, the instantaneous velocity of a Brownian particle (a glass microsphere trapped in air with optical tweezers) was measured successfully. The velocity data verified the Maxwell–Boltzmann velocity distribution, and the equipartition theorem for a Brownian particle.
Statistical mechanics theories | Brownian motion | Wikipedia | 473 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
Einstein's theory
There are two parts to Einstein's theory: the first part consists in the formulation of a diffusion equation for Brownian particles, in which the diffusion coefficient is related to the mean squared displacement of a Brownian particle, while the second part consists in relating the diffusion coefficient to measurable physical quantities. In this way Einstein was able to determine the size of atoms, and how many atoms there are in a mole, or the molecular weight in grams, of a gas. In accordance to Avogadro's law, this volume is the same for all ideal gases, which is 22.414 liters at standard temperature and pressure. The number of atoms contained in this volume is referred to as the Avogadro number, and the determination of this number is tantamount to the knowledge of the mass of an atom, since the latter is obtained by dividing the molar mass of the gas by the Avogadro constant.
The first part of Einstein's argument was to determine how far a Brownian particle travels in a given time interval. Classical mechanics is unable to determine this distance because of the enormous number of bombardments a Brownian particle will undergo, roughly of the order of 1014 collisions per second.
He regarded the increment of particle positions in time in a one-dimensional (x) space (with the coordinates chosen so that the origin lies at the initial position of the particle) as a random variable () with some probability density function (i.e., is the probability density for a jump of magnitude , i.e., the probability density of the particle incrementing its position from to in the time interval ). Further, assuming conservation of particle number, he expanded the number density (number of particles per unit volume around ) at time in a Taylor series,
where the second equality is by definition of . The integral in the first term is equal to one by the definition of probability, and the second and other even terms (i.e. first and other odd moments) vanish because of space symmetry. What is left gives rise to the following relation:
Where the coefficient after the Laplacian, the second moment of probability of displacement , is interpreted as mass diffusivity D:
Then the density of Brownian particles at point at time satisfies the diffusion equation:
Assuming that N particles start from the origin at the initial time t = 0, the diffusion equation has the solution | Brownian motion | Wikipedia | 493 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
This expression (which is a normal distribution with the mean and variance usually called Brownian motion ) allowed Einstein to calculate the moments directly. The first moment is seen to vanish, meaning that the Brownian particle is equally likely to move to the left as it is to move to the right. The second moment is, however, non-vanishing, being given by
This equation expresses the mean squared displacement in terms of the time elapsed and the diffusivity. From this expression Einstein argued that the displacement of a Brownian particle is not proportional to the elapsed time, but rather to its square root. His argument is based on a conceptual switch from the "ensemble" of Brownian particles to the "single" Brownian particle: we can speak of the relative number of particles at a single instant just as well as of the time it takes a Brownian particle to reach a given point.
The second part of Einstein's theory relates the diffusion constant to physically measurable quantities, such as the mean squared displacement of a particle in a given time interval. This result enables the experimental determination of the Avogadro number and therefore the size of molecules. Einstein analyzed a dynamic equilibrium being established between opposing forces. The beauty of his argument is that the final result does not depend upon which forces are involved in setting up the dynamic equilibrium.
In his original treatment, Einstein considered an osmotic pressure experiment, but the same conclusion can be reached in other ways.
Consider, for instance, particles suspended in a viscous fluid in a gravitational field. Gravity tends to make the particles settle, whereas diffusion acts to homogenize them, driving them into regions of smaller concentration. Under the action of gravity, a particle acquires a downward speed of , where is the mass of the particle, is the acceleration due to gravity, and is the particle's mobility in the fluid. George Stokes had shown that the mobility for a spherical particle with radius is , where is the dynamic viscosity of the fluid. In a state of dynamic equilibrium, and under the hypothesis of isothermal fluid, the particles are distributed according to the barometric distribution
where is the difference in density of particles separated by a height difference, of , is the Boltzmann constant (the ratio of the universal gas constant, , to the Avogadro constant, ), and is the absolute temperature. | Brownian motion | Wikipedia | 477 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
Dynamic equilibrium is established because the more that particles are pulled down by gravity, the greater the tendency for the particles to migrate to regions of lower concentration. The flux is given by Fick's law,
where . Introducing the formula for , we find that
In a state of dynamical equilibrium, this speed must also be equal to . Both expressions for are proportional to , reflecting that the derivation is independent of the type of forces considered. Similarly, one can derive an equivalent formula for identical charged particles of charge in a uniform electric field of magnitude , where is replaced with the electrostatic force . Equating these two expressions yields the Einstein relation for the diffusivity, independent of or or other such forces:
Here the first equality follows from the first part of Einstein's theory, the third equality follows from the definition of the Boltzmann constant as , and the fourth equality follows from Stokes's formula for the mobility. By measuring the mean squared displacement over a time interval along with the universal gas constant , the temperature , the viscosity , and the particle radius , the Avogadro constant can be determined.
The type of dynamical equilibrium proposed by Einstein was not new. It had been pointed out previously by J. J. Thomson in his series of lectures at Yale University in May 1903 that the dynamic equilibrium between the velocity generated by a concentration gradient given by Fick's law and the velocity due to the variation of the partial pressure caused when ions are set in motion "gives us a method of determining Avogadro's constant which is independent of any hypothesis as to the shape or size of molecules, or of the way in which they act upon each other". | Brownian motion | Wikipedia | 340 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
An identical expression to Einstein's formula for the diffusion coefficient was also found by Walther Nernst in 1888 in which he expressed the diffusion coefficient as the ratio of the osmotic pressure to the ratio of the frictional force and the velocity to which it gives rise. The former was equated to the law of van 't Hoff while the latter was given by Stokes's law. He writes for the diffusion coefficient , where is the osmotic pressure and is the ratio of the frictional force to the molecular viscosity which he assumes is given by Stokes's formula for the viscosity. Introducing the ideal gas law per unit volume for the osmotic pressure, the formula becomes identical to that of Einstein's. The use of Stokes's law in Nernst's case, as well as in Einstein and Smoluchowski, is not strictly applicable since it does not apply to the case where the radius of the sphere is small in comparison with the mean free path.
At first, the predictions of Einstein's formula were seemingly refuted by a series of experiments by Svedberg in 1906 and 1907, which gave displacements of the particles as 4 to 6 times the predicted value, and by Henri in 1908 who found displacements 3 times greater than Einstein's formula predicted. But Einstein's predictions were finally confirmed in a series of experiments carried out by Chaudesaigues in 1908 and Perrin in 1909. The confirmation of Einstein's theory constituted empirical progress for the kinetic theory of heat. In essence, Einstein showed that the motion can be predicted directly from the kinetic model of thermal equilibrium. The importance of the theory lay in the fact that it confirmed the kinetic theory's account of the second law of thermodynamics as being an essentially statistical law.
Smoluchowski model
Smoluchowski's theory of Brownian motion starts from the same premise as that of Einstein and derives the same probability distribution for the displacement of a Brownian particle along the in time . He therefore gets the same expression for the mean squared displacement: However, when he relates it to a particle of mass moving at a velocity which is the result of a frictional force governed by Stokes's law, he finds | Brownian motion | Wikipedia | 458 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
where is the viscosity coefficient, and is the radius of the particle. Associating the kinetic energy with the thermal energy , the expression for the mean squared displacement is times that found by Einstein. The fraction 27/64 was commented on by Arnold Sommerfeld in his necrology on Smoluchowski: "The numerical coefficient of Einstein, which differs from Smoluchowski by 27/64 can only be put in doubt."
Smoluchowski attempts to answer the question of why a Brownian particle should be displaced by bombardments of smaller particles when the probabilities for striking it in the forward and rear directions are equal.
If the probability of gains and losses follows a binomial distribution,
with equal probabilities of 1/2, the mean total gain is
If is large enough so that Stirling's approximation can be used in the form
then the expected total gain will be
showing that it increases as the square root of the total population.
Suppose that a Brownian particle of mass is surrounded by lighter particles of mass which are traveling at a speed . Then, reasons Smoluchowski, in any collision between a surrounding and Brownian particles, the velocity transmitted to the latter will be . This ratio is of the order of . But we also have to take into consideration that in a gas there will be more than 1016 collisions in a second, and even greater in a liquid where we expect that there will be 1020 collision in one second. Some of these collisions will tend to accelerate the Brownian particle; others will tend to decelerate it. If there is a mean excess of one kind of collision or the other to be of the order of 108 to 1010 collisions in one second, then velocity of the Brownian particle may be anywhere between . Thus, even though there are equal probabilities for forward and backward collisions there will be a net tendency to keep the Brownian particle in motion, just as the ballot theorem predicts. | Brownian motion | Wikipedia | 399 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
These orders of magnitude are not exact because they don't take into consideration the velocity of the Brownian particle, , which depends on the collisions that tend to accelerate and decelerate it. The larger is, the greater will be the collisions that will retard it so that the velocity of a Brownian particle can never increase without limit. Could such a process occur, it would be tantamount to a perpetual motion of the second type. And since equipartition of energy applies, the kinetic energy of the Brownian particle, will be equal, on the average, to the kinetic energy of the surrounding fluid particle,
In 1906 Smoluchowski published a one-dimensional model to describe a particle undergoing Brownian motion. The model assumes collisions with where is the test particle's mass and the mass of one of the individual particles composing the fluid. It is assumed that the particle collisions are confined to one dimension and that it is equally probable for the test particle to be hit from the left as from the right. It is also assumed that every collision always imparts the same magnitude of . If is the number of collisions from the right and the number of collisions from the left then after collisions the particle's velocity will have changed by . The multiplicity is then simply given by:
and the total number of possible states is given by . Therefore, the probability of the particle being hit from the right times is:
As a result of its simplicity, Smoluchowski's 1D model can only qualitatively describe Brownian motion. For a realistic particle undergoing Brownian motion in a fluid, many of the assumptions don't apply. For example, the assumption that on average occurs an equal number of collisions from the right as from the left falls apart once the particle is in motion. Also, there would be a distribution of different possible s instead of always just one in a realistic situation.
Langevin equation
The diffusion equation yields an approximation of the time evolution of the probability density function associated with the position of the particle going under a Brownian movement under the physical definition. The approximation is valid on short timescales.
The time evolution of the position of the Brownian particle over all time scales described using the Langevin equation, an equation that involves a random force field representing the effect of the thermal fluctuations of the solvent on the particle. In Langevin dynamics and Brownian dynamics, the Langevin equation is used to efficiently simulate the dynamics of molecular systems that exhibit a strong Brownian component. | Brownian motion | Wikipedia | 509 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
Astrophysics: star motion within galaxies
In stellar dynamics, a massive body (star, black hole, etc.) can experience Brownian motion as it responds to gravitational forces from surrounding stars. The rms velocity of the massive object, of mass , is related to the rms velocity of the background stars by
where is the mass of the background stars. The gravitational force from the massive object causes nearby stars to move faster than they otherwise would, increasing both and . The Brownian velocity of Sgr A*, the supermassive black hole at the center of the Milky Way galaxy, is predicted from this formula to be less than 1 km s−1.
Mathematics
In mathematics, Brownian motion is described by the Wiener process, a continuous-time stochastic process named in honor of Norbert Wiener. It is one of the best known Lévy processes (càdlàg stochastic processes with stationary independent increments) and occurs frequently in pure and applied mathematics, economics and physics.
The Wiener process is characterized by four facts:
is almost surely continuous
has independent increments
denotes the normal distribution with expected value and variance . The condition that it has independent increments means that if then and are independent random variables. In addition, for some filtration is measurable for all
An alternative characterisation of the Wiener process is the so-called Lévy characterisation that says that the Wiener process is an almost surely continuous martingale with and quadratic variation
A third characterisation is that the Wiener process has a spectral representation as a sine series whose coefficients are independent random variables. This representation can be obtained using the Kosambi–Karhunen–Loève theorem.
The Wiener process can be constructed as the scaling limit of a random walk, or other discrete-time stochastic processes with stationary independent increments. This is known as Donsker's theorem. Like the random walk, the Wiener process is recurrent in one or two dimensions (meaning that it returns almost surely to any fixed neighborhood of the origin infinitely often) whereas it is not recurrent in dimensions three and higher. Unlike the random walk, it is scale invariant.
A d-dimensional Gaussian free field has been described as "a d-dimensional-time analog of Brownian motion."
Statistics
The Brownian motion can be modeled by a random walk.
In the general case, Brownian motion is a Markov process and described by stochastic integral equations.
Lévy characterisation | Brownian motion | Wikipedia | 498 | 4436 | https://en.wikipedia.org/wiki/Brownian%20motion | Physical sciences | Thermodynamics | Physics |
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