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# List of presidents of the United States - Wikipedia
The president of the United States is the head of state and head of government of the United States, indirectly elected to a four-year term via the Electoral College. Under the U.S. Constitution, the officeholder leads the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces.
The first president, George Washington, won a unanimous vote of the Electoral College. The incumbent president is Donald Trump, who assumed office on January 20, 2025. Since the office was established in 1789, 45 men have served in 47 presidencies; the discrepancy arises from two individuals elected to nonconsecutive terms: Grover Cleveland is counted as both the 22nd and 24th president, while Trump is counted as both the 45th and 47th president.
The presidency of William Henry Harrison, who died 31 days after taking office in 1841, was the shortest in American history. Franklin D. Roosevelt served the longest, over twelve years, before dying early in his fourth term in 1945. He is the only U.S. president to have served more than two terms. Since the ratification of the Twenty-second Amendment to the United States Constitution in 1951, no person may be elected president more than twice, and no one who has served more than two years of a term to which someone else was elected may be elected more than once.
Four presidents died in office of natural causes (William Henry Harrison, Zachary Taylor, Warren G. Harding, and Franklin D. Roosevelt), four were assassinated (Abraham Lincoln, James A. Garfield, William McKinley, and John F. Kennedy), and one resigned (Richard Nixon, facing impeachment and removal from office). John Tyler was the first vice president to assume the presidency during a presidential term, setting the precedent that a vice president who does so becomes the fully functioning president with a new, distinct administration.
Throughout most of its history, American politics has been dominated by political parties. The Constitution is silent on the issue of political parties, and at the time it came into force in 1789, no organized parties existed. Soon after the 1st Congress convened, political factions began rallying around dominant Washington administration officials, such as Alexander Hamilton and Thomas Jefferson. Concerned about the capacity of political parties to destroy the fragile unity holding the nation together, Washington remained unaffiliated with any political faction or party throughout his eight-year presidency. He was, and remains, the only U.S. president who never affiliated with a political party.
## Presidents
| No. | Portrait | Name (birth–death) |
Term | Party | Election | Vice President | |
|---|---|---|---|---|---|---|---|
| 1 | George Washington(1732–1799) |
April 30, 1789 – March 4, 1797 |
Unaffiliated
|
1788–89 1792 |
John Adams | ||
| 2 | John Adams(1735–1826) |
March 4, 1797 – March 4, 1801 |
Federalist | 1796 | Thomas Jefferson | ||
| 3 | Thomas Jefferson(1743–1826) |
March 4, 1801 – March 4, 1809 |
Democratic- Republican |
1800 1804 |
Aaron Burr George Clinton | ||
| 4 | James Madison(1751–1836) |
March 4, 1809 – March 4, 1817 |
Democratic- Republican |
1808 1812 |
George ClintonVacant afterApril 20, 1812 Elbridge Gerry Vacant after
November 23, 1814 | ||
| 5 | James Monroe(1758–1831) |
March 4, 1817 – March 4, 1825 |
Democratic- Republican |
1816 1820 |
Daniel D. Tompkins | ||
| 6 | John Quincy Adams(1767–1848) |
March 4, 1825 – March 4, 1829 |
Democratic- Republican National Republican |
1824 | John C. Calhoun | ||
| 7 | Andrew Jackson(1767–1845) |
March 4, 1829 – March 4, 1837 |
Democratic | 1828 1832 |
John C. CalhounVacant afterDecember 28, 1832 Martin Van Buren | ||
| 8 | Martin Van Buren(1782–1862) |
March 4, 1837 – March 4, 1841 |
Democratic | 1836 | Richard Mentor Johnson | ||
| 9 | William Henry Harrison(1773–1841) |
March 4, 1841 – April 4, 1841 |
Whig | 1840 | John Tyler | ||
| 10 | John Tyler(1790–1862) |
April 4, 1841 – March 4, 1845 |
WhigUnaffiliated
|
– | Vacant throughout
presidency | ||
| 11 | James K. Polk(1795–1849) |
March 4, 1845 – March 4, 1849 |
Democratic | 1844 | George M. Dallas | ||
| 12 | Zachary Taylor(1784–1850) |
March 4, 1849 – July 9, 1850 |
Whig | 1848 | Millard Fillmore | ||
| 13 | Millard Fillmore(1800–1874) |
July 9, 1850 – March 4, 1853 |
Whig | – | Vacant throughout
presidency | ||
| 14 | Franklin Pierce(1804–1869) |
March 4, 1853 – March 4, 1857 |
Democratic | 1852 | William R. KingVacant after
April 18, 1853 | ||
| 15 | James Buchanan(1791–1868) |
March 4, 1857 – March 4, 1861 |
Democratic | 1856 | John C. Breckinridge | ||
| 16 | Abraham Lincoln(1809–1865) |
March 4, 1861 – April 15, 1865 |
Republican National Union |
1860 1864 |
Hannibal Hamlin Andrew Johnson | ||
| 17 | Andrew Johnson(1808–1875) |
April 15, 1865 – March 4, 1869 |
National Union Democratic |
– | Vacant throughout
presidency | ||
| 18 | Ulysses S. Grant(1822–1885) |
March 4, 1869 – March 4, 1877 |
Republican | 1868 1872 |
Schuyler Colfax Henry Wilson Vacant after
November 22, 1875 | ||
| 19 | Rutherford B. Hayes(1822–1893) |
March 4, 1877 – March 4, 1881 |
Republican | 1876 | William A. Wheeler | ||
| 20 | James A. Garfield(1831–1881) |
March 4, 1881 – September 19, 1881 |
Republican | 1880 | Chester A. Arthur | ||
| 21 | Chester A. Arthur(1829–1886) |
September 19, 1881 – March 4, 1885 |
Republican | – | Vacant throughout
presidency | ||
| 22 | Grover Cleveland(1837–1908) |
March 4, 1885 – March 4, 1889 |
Democratic | 1884 | Thomas A. HendricksVacant after
November 25, 1885 | ||
| 23 | Benjamin Harrison(1833–1901) |
March 4, 1889 – March 4, 1893 |
Republican | 1888 | Levi P. Morton | ||
| 24 | Grover Cleveland(1837–1908) |
March 4, 1893 – March 4, 1897 |
Democratic | 1892 | Adlai Stevenson I | ||
| 25 | William McKinley(1843–1901) |
March 4, 1897 – September 14, 1901 |
Republican | 1896 1900 |
Garret HobartVacant afterNovember 21, 1899 Theodore Roosevelt | ||
| 26 | Theodore Roosevelt(1858–1919) |
September 14, 1901 – March 4, 1909 |
Republican | – 1904 |
Vacant throughMarch 4, 1905 Charles W. Fairbanks | ||
| 27 | William Howard Taft(1857–1930) |
March 4, 1909 – March 4, 1913 |
Republican | 1908 | James S. ShermanVacant after
October 30, 1912 | ||
| 28 | Woodrow Wilson(1856–1924) |
March 4, 1913 – March 4, 1921 |
Democratic | 1912 1916 |
Thomas R. Marshall | ||
| 29 | Warren G. Harding(1865–1923) |
March 4, 1921 – August 2, 1923 |
Republican | 1920 | Calvin Coolidge | ||
| 30 | Calvin Coolidge(1872–1933) |
August 2, 1923 – March 4, 1929 |
Republican | – 1924 |
Vacant throughMarch 4, 1925 Charles G. Dawes | ||
| 31 | Herbert Hoover(1874–1964) |
March 4, 1929 – March 4, 1933 |
Republican | 1928 | Charles Curtis | ||
| 32 | Franklin D. Roosevelt(1882–1945) |
March 4, 1933 – April 12, 1945 |
Democratic | 1932 1936 1940 1944 |
John Nance Garner Henry A. Wallace Harry S. Truman | ||
| 33 | Harry S. Truman(1884–1972) |
April 12, 1945 – January 20, 1953 |
Democratic | – 1948 |
Vacant throughJanuary 20, 1949 Alben W. Barkley | ||
| 34 | Dwight D. Eisenhower(1890–1969) |
January 20, 1953 – January 20, 1961 |
Republican | 1952 1956 |
Richard Nixon | ||
| 35 | John F. Kennedy(1917–1963) |
January 20, 1961 – November 22, 1963 |
Democratic | 1960 | Lyndon B. Johnson | ||
| 36 | Lyndon B. Johnson(1908–1973) |
November 22, 1963 – January 20, 1969 |
Democratic | – 1964 |
Vacant throughJanuary 20, 1965 Hubert Humphrey | ||
| 37 | Richard Nixon(1913–1994) |
January 20, 1969 – August 9, 1974 |
Republican | 1968 1972 |
Spiro AgnewVacant:October 10 – December 6, 1973 Gerald Ford | ||
| 38 | Gerald Ford(1913–2006) |
August 9, 1974 – January 20, 1977 |
Republican | – | Vacant throughDecember 19, 1974 Nelson Rockefeller | ||
| 39 | Jimmy Carter(1924–2024) |
January 20, 1977 – January 20, 1981 |
Democratic | 1976 | Walter Mondale | ||
| 40 | Ronald Reagan(1911–2004) |
January 20, 1981 – January 20, 1989 |
Republican | 1980 1984 |
George H. W. Bush | ||
| 41 | George H. W. Bush(1924–2018) |
January 20, 1989 – January 20, 1993 |
Republican | 1988 | Dan Quayle | ||
| 42 | Bill Clinton(b. 1946) |
January 20, 1993 – January 20, 2001 |
Democratic | 1992 1996 |
Al Gore | ||
| 43 | George W. Bush(b. 1946) |
January 20, 2001 – January 20, 2009 |
Republican | 2000 2004 |
Dick Cheney | ||
| 44 | Barack Obama(b. 1961) |
January 20, 2009 – January 20, 2017 |
Democratic | 2008 2012 |
Joe Biden | ||
| 45 | Donald Trump(b. 1946) |
January 20, 2017 – January 20, 2021 |
Republican | 2016 | Mike Pence | ||
| 46 | Joe Biden(b. 1942) |
January 20, 2021 – January 20, 2025 |
Democratic | 2020 | Kamala Harris | ||
| 47 | Donald Trump(b. 1946) |
January 20, 2025 – Incumbent
|
Republican | 2024 | JD Vance |
## See also
- Acting President of the United States
- Founding Fathers of the United States
- Historical rankings of presidents of the United States
- List of vice presidents of the United States
- President of the Continental Congress
## Notes
## Works cited
## External links
- Media related to President of the United States at Wikimedia Commons
- Quotations related to List of presidents of the United States at Wikiquote
|
# Animal - Wikipedia
| Animals | |
|---|---|
| Scientific classification | |
| Domain: | Eukaryota |
Clade:
|
Amorphea |
Clade:
|
Obazoa |
Clade:
|
Opisthokonta |
Clade:
|
Holozoa |
Clade:
|
Filozoa |
Clade:
|
Choanozoa |
| Kingdom: | Animalia Linnaeus, 1758 |
| Major groups | |
| |
| Synonyms | |
|
**Animals** are multicellular, eukaryotic organisms in the biological kingdom **Animalia** (consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor. Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology, and the study of animal behaviour is known as ethology.
The animal kingdom is divided into five major clades, namely Porifera, Ctenophora, Placozoa, Cnidaria and Bilateria. Most living animal species belong to the clade Bilateria, a highly proliferative clade whose members have a bilaterally symmetric and significantly cephalised body plan, and the vast majority of bilaterians belong to two large clades: the protostomes, which includes organisms such as arthropods, molluscs, flatworms, annelids and nematodes; and the deuterostomes, which include echinoderms, hemichordates and chordates, the latter of which contains the vertebrates. The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria.
Animals first appeared in the fossil record in the late Cryogenian period and diversified in the subsequent Ediacaran period in what is known as the Avalon explosion. Earlier evidence of animals is still controversial; the sponge-like organism *Otavia* has been dated back to the Tonian period at the start of the Neoproterozoic, but its identity as an animal is heavily contested. Nearly all modern animal phyla first appeared in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. Common to all living animals, 6,331 groups of genes have been identified that may have arisen from a single common ancestor that lived about 650 Mya during the Cryogenian period.
Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his *Systema Naturae*, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular **Metazoa** (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between taxa.
Humans make use of many other animal species for food (including meat, eggs, and dairy products), for materials (such as leather, fur, and wool), as pets and as working animals for transportation, and services. Dogs, the first domesticated animal, have been used in hunting, in security and in warfare, as have horses, pigeons and birds of prey; while other terrestrial and aquatic animals are hunted for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution, having appeared in cave arts and totems since the earliest times, and are frequently featured in mythology, religion, arts, literature, heraldry, politics, and sports.
## Etymology
The word *animal* comes from the Latin noun *animal* of the same meaning, which is itself derived from Latin *animalis* 'having breath or soul'. The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term *animal* is often used to refer only to nonhuman animals. The term *metazoa* is derived from Ancient Greek μετα *meta* 'after' (in biology, the prefix *meta-* stands for 'later') and ζῷᾰ *zōia* 'animals', plural of ζῷον *zōion* 'animal'.
## Characteristics
Animals have several characteristics that they share with other living things. Animals are eukaryotic, multicellular, and aerobic, as are plants and fungi. Unlike plants and algae, which produce their own food, animals cannot produce their own food a feature they share with fungi. Animals ingest organic material and digest it internally.
### Structural features
Animals have structural characteristics that set them apart from all other living things:
- cells surrounded by an extracellular matrix composed of
- collagen and
- elastic glycoproteins
- motility i.e. able to spontaneously move their bodies during at least part of their life cycle.
- a blastula stage during embryonic development
Typically, there is an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
### Development
Animal development is controlled by Hox genes, which signal the times and places to develop structures such as body segments and limbs.
During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised into specialised tissues and organs, making the formation of complex structures possible, and allowing cells to be differentiated. The extracellular matrix may be calcified, forming structures such as shells, bones, and spicules. In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.
### Reproduction
Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between them. These germ layers then differentiate to form tissues and organs.
Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding.
Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in *Hydra* and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.
## Ecology
Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material. Such groupings include carnivores (further divided into subcategories such as piscivores, insectivores, ovivores, etc.), herbivores (subcategorised into folivores, graminivores, frugivores, granivores, nectarivores, algivores, etc.), omnivores, fungivores, scavengers/detritivores, and parasites. Interactions between animals of each biome form complex food webs within that ecosystem. In carnivorous or omnivorous species, predation is a consumer–resource interaction where the predator feeds on another organism, its prey, who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/competitive coevolutions. Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours, such as hawksbill sea turtles which mainly eat sponges.
Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers) through photosynthesis. Herbivores, as primary consumers, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidise carbohydrates, lipids, proteins and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion. Some benthic animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter produced through chemosynthesis (via oxidising inorganic compounds such as hydrogen sulfide) by archaea and bacteria.
Animals evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician. Vertebrates such as the lobe-finned fish *Tiktaalik* started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms. Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F) or in the most extreme cold deserts of continental Antarctica.
The collective global geomorphic influence of animals on the processes shaping the Earth's surface remains largely understudied, with most studies limited to individual species and well-known exemplars.
## Diversity
### Size
The blue whale (*Balaenoptera musculus*) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant (*Loxodonta africana*), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as *Argentinosaurus*, which may have weighed as much as 73 tonnes, and *Supersaurus* which may have reached 39 metres. Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 μm, and one of the smallest species (*Myxobolus shekel*) is no more than 8.5 μm when fully grown.
### Numbers and habitats of major phyla
The following table lists estimated numbers of described extant species for the major animal phyla, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.
| Phylum | Example | Species | Land | Sea | Freshwater | Free-living | Parasitic |
|---|---|---|---|---|---|---|---|
Arthropoda
|
1,257,000 | Yes 1,000,000 (insects) |
Yes >40,000 (Malac- ostraca) |
Yes 94,000 | Yes | Yes >45,000 | |
Mollusca
|
85,000 107,000 |
35,000 | 60,000 | 5,000 12,000 |
Yes | >5,600 | |
Chordata
|
>70,000 | 23,000 | 13,000 | 18,000 9,000 |
Yes | 40 (catfish) | |
Platyhelminthes
|
29,500 | Yes | Yes | 1,300 | Yes 3,000–6,500 |
>40,000 4,000–25,000 | |
Nematoda
|
25,000 | Yes (soil) | 4,000 | 2,000 | 11,000 | 14,000 | |
Annelida
|
17,000 | Yes (soil) | Yes | 1,750 | Yes | 400 | |
Cnidaria
|
16,000 | Yes | Few | Yes | >1,350 (Myxozoa) | ||
Porifera
|
10,800 | Yes | 200–300 | Yes | Yes | ||
Echinodermata
|
7,500 | 7,500 | Yes | ||||
Bryozoa
|
6,000 | Yes | 60–80 | Yes | |||
Rotifera
|
2,000 | >400 | 2,000 | Yes | Yes | ||
Nemertea
|
1,350 | Yes | Yes | Yes | |||
Tardigrada
|
1,335 | Yes (moist plants) |
Yes | Yes | Yes |
## Evolutionary origin
Evidence of animals is found as long ago as the Cryogenian period. 24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges and pelagophyte algae. Its likely origin is from sponges based on molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a Phanerozoic origin, while analyses of sponges recover a Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record.
The first body fossils of animals appear in the Ediacaran, represented by forms such as *Charnia* and *Spriggina*. It had long been doubted whether these fossils truly represented animals, but the discovery of the animal lipid cholesterol in fossils of *Dickinsonia* establishes their nature. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialised for aerobic metabolism they became fully dependent on oxygen in their environments.
Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 539 million years ago, in beds such as the Burgess shale. Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory *Anomalocaris*. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously. That view is supported by the discovery of *Auroralumina attenboroughii*, the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from Charnwood Forest, England. It is thought to be one of the earliest predators, catching small prey with its nematocysts as modern cnidarians do.
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges.
Trace fossils such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced by the giant single-celled protist *Gromia sphaerica*, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals. Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures.
## Phylogeny
### External phylogeny
Animals are monophyletic, meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates, with which they form the Choanozoa. Ros-Rocher and colleagues (2021) trace the origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines. The animal clade had certainly originated by 650 mya, and may have come into being as much as 800 mya, based on molecular clock evidence for different phyla.
### Internal phylogeny
The relationships at the base of the animal tree have been debated. Other than Ctenophora, the Bilateria and Cnidaria are the only groups with symmetry, and other evidence shows they are closely related. In addition to sponges, Placozoa has no symmetry and was often considered a "missing link" between protists and multicellular animals. The presence of hox genes in Placozoa shows that they were once more complex.
The Porifera (sponges) have long been assumed to be sister to the rest of the animals, but there is evidence that the Ctenophora may be in that position. Molecular phylogenetics has supported both the sponge-sister and ctenophore-sister hypotheses. In 2017, Roberto Feuda and colleagues, using amino acid differences, presented both, with the following cladogram for the sponge-sister view that they supported (their ctenophore-sister tree simply interchanging the places of ctenophores and sponges):
Conversely, a 2023 study by Darrin Schultz and colleagues uses ancient gene linkages to construct the following ctenophore-sister phylogeny:
### Non-bilaterians
Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a sister clade to all other animals. Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are. Sponges lack the complex organisation found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals. They typically feed by drawing in water through pores, filtering out small particles of food.
The Ctenophora and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into discrete organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm.
The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae. Their phylogeny is poorly defined, and under active research.
### Bilateria
The remaining animals, the great majority—comprising some 29 phyla and over a million species—form the Bilateria clade, which have a bilaterally symmetric body plan. The Bilateria are triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and in the Nephrozoa there is an internal body cavity, a coelom or pseudocoelom. These animals have a head end (anterior) and a tail end (posterior), a back (dorsal) surface and a belly (ventral) surface, and a left and a right side. A modern consensus phylogenetic tree for the Bilateria is shown below.
Bilateria
|
|
Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, over evolutionary time, descendant spaces have evolved which have lost one or more of each of these characteristics. For example, adult echinoderms are radially symmetric (unlike their larvae), while some parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the protostomes and the deuterostomes. It is often suggested that the basalmost bilaterians are the Xenacoelomorpha, with all other bilaterians belonging to the subclade Nephrozoa. However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to Ambulacraria than to other bilaterians.
#### Protostomes and deuterostomes
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.
The main deuterostome phyla are the Ambulacraria and the Chordata. Ambulacraria are exclusively marine and include acorn worms, starfish, sea urchins, and sea cucumbers. The chordates are dominated by the vertebrates (animals with backbones), which consist of fishes, amphibians, reptiles, birds, and mammals.
The protostomes include the Ecdysozoa, named after their shared trait of ecdysis, growth by moulting, Among the largest ecdysozoan phyla are the arthropods and the nematodes. The rest of the protostomes are in the Spiralia, named for their pattern of developing by spiral cleavage in the early embryo. Major spiralian phyla include the annelids and molluscs.
## History of classification
In the classical era, Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then arranged on a scale from man (with blood, two legs, rational soul) down through the live-bearing tetrapods (with blood, four legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously generating creatures like sponges (no blood, no legs, vegetable soul). Aristotle was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.
In 1758, Carl Linnaeus created the first hierarchical classification in his *Systema Naturae*. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then, the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes *une espèce de chaos* ('a chaotic mess') and split the group into three new phyla: worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his *Philosophie Zoologique*, Lamarck had created nine phyla apart from vertebrates (where he still had four phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.
In his 1817 * Le Règne Animal*, Georges Cuvier used comparative anatomy to group the animals into four
*embranchements*('branches' with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes (radiata) (echinoderms, cnidaria and other forms). This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860.
In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.
## In human culture
### Practical uses
The human population exploits a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially. Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.
Invertebrates including cephalopods, crustaceans, insects—principally bees and silkworms—and bivalve or gastropod molluscs are hunted or farmed for food, fibres. Chickens, cattle, sheep, pigs, and other animals are raised as livestock for meat across the world. Animal fibres such as wool and silk are used to make textiles, while animal sinews have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats. Dyestuffs including carmine (cochineal), shellac, and kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture.
Animals such as the fruit fly *Drosophila melanogaster* serve a major role in science as experimental models. Animals have been used to create vaccines since their discovery in the 18th century. Some medicines such as the cancer drug trabectedin are based on toxins or other molecules of animal origin.
People have used hunting dogs to help chase down and retrieve animals, and birds of prey to catch birds and mammals, while tethered cormorants have been used to catch fish. Poison dart frogs have been used to poison the tips of blowpipe darts. A wide variety of animals are kept as pets, from invertebrates such as tarantulas, octopuses, and praying mantises, reptiles such as snakes and chameleons, and birds including canaries, parakeets, and parrots all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits. There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.
A wide variety of terrestrial and aquatic animals are hunted for sport.
### Symbolic uses
The signs of the Western and Chinese zodiacs are based on animals. In China and Japan, the butterfly has been seen as the personification of a person's soul, and in classical representation the butterfly is also the symbol of the soul.
Animals have been the subjects of art from the earliest times, both historical, as in ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 *The Rhinoceros*, and George Stubbs's c. 1762 horse portrait *Whistlejacket*. Insects, birds and mammals play roles in literature and film, such as in giant bug movies.
Animals including insects and mammals feature in mythology and religion. The scarab beetle was sacred in ancient Egypt, and the cow is sacred in Hinduism. Among other mammals, deer, horses, lions, bats, bears, and wolves are the subjects of myths and worship.
## See also
- Animal coloration
- Ethology
- Lists of organisms by population
- World Animal Day, observed on 4 October
## Notes
## External links
- Media related to Animals at Wikimedia Commons
- Data related to Animal at Wikispecies
- Tree of Life Project. Archived 12 June 2011 at the Wayback Machine
- Animal Diversity Web – University of Michigan's database of animals
- Wildscreen Arkive – multimedia database of endangered/protected species
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# Eukaryote - Wikipedia
| Eukaryotes | |
|---|---|
| Scientific classification | |
| Domain: | Eukaryota (Chatton, 1925) Whittaker & Margulis, 1978 |
| Major subdivisions | |
| Synonyms | |
The **eukaryotes** (*yoo-KARR-ee-ohts, -əts*) constitute the domain of **Eukaryota** or **Eukarya**, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of life forms alongside the two groups of prokaryotes: the Bacteria and the Archaea. Eukaryotes represent a small minority of the number of organisms, but given their generally much larger size, their collective global biomass is much larger than that of prokaryotes.
The eukaryotes emerged within the archaeal kingdom Promethearchaeati and its sole phylum Promethearchaeota. This implies that there are only two domains of life, Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic, likely as flagellated cells. The leading evolutionary theory is they were created by symbiogenesis between an anaerobic Promethearchaeati archaean and an aerobic proteobacterium, which formed the mitochondria. A second episode of symbiogenesis with a cyanobacterium created the plants, with chloroplasts.
Eukaryotic cells contain membrane-bound organelles such as the nucleus, the endoplasmic reticulum, and the Golgi apparatus. Eukaryotes may be either unicellular or multicellular. In comparison, prokaryotes are typically unicellular. Unicellular eukaryotes are sometimes called protists. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion (fertilization).
The word *eukaryote* is derived from the Greek words "eu" (εὖ) meaning "true" or "good" and "karyon" (κάρυον) meaning "nut" or "kernel", referring to the nucleus of a cell.
Eukaryotes are organisms that range from microscopic single cells, such as picozoans under 3 micrometres across, to animals like the blue whale, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long, or plants like the coast redwood, up to 120 metres (390 ft) tall. Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 feet (61 m) long. The multicellular eukaryotes include the animals, plants, and fungi, but again, these groups too contain many unicellular species. Eukaryotic cells are typically much larger than those of prokaryotes—the bacteria and the archaea—having a volume of around 10,000 times greater. Eukaryotes represent a small minority of the number of organisms, but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth.
The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms. Multicellularity in some form has evolved independently at least 25 times within the eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds, have evolved within only six eukaryotic lineages: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants. Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.
The defining feature of eukaryotes is that their cells have a well-defined, membrane-bound nuclei, distinguishing them from prokaryotes that lack such a structure. Eukaryotic cells have a variety of internal membrane-bound structures, called organelles, and a cytoskeleton which defines the cell's organization and shape. The nucleus stores the cell's DNA, which is divided into linear bundles called chromosomes; these are separated into two matching sets by a microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis.
Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis. The eukaryotic signature proteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton, the complex transcription machinery, the membrane-sorting systems, the nuclear pore, and some enzymes in the biochemical pathways.
Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. Some cell products can leave in a vesicle through exocytosis.
The nucleus is surrounded by a double membrane known as the nuclear envelope, with nuclear pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, which is involved in protein transport and maturation. It includes the rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and their contents further modified in stacks of flattened vesicles (cisternae), the Golgi apparatus.
Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in the cytoplasm.
Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell", for its function providing energy by oxidising sugars or fats to produce the energy-storing molecule ATP. Mitochondria have two surrounding membranes, each a phospholipid bilayer, the inner of which is folded into invaginations called cristae where aerobic respiration takes place.
Mitochondria contain their own DNA, which has close structural similarities to bacterial DNA, from which it originated, and which encodes rRNA and tRNA genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA.
Some eukaryotes, such as the metamonads *Giardia* and *Trichomonas*, and the amoebozoan *Pelomyxa*, appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes, having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. It is thought that mitochondria developed from prokaryotic cells which became endosymbionts living inside eukaryotes.
Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts, in this case cyanobacteria. They usually take the form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose) through photosynthesis. Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty, occurs in many types of modern eukaryotic organisms.
The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable the cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins, including α-actinin, fimbrin, and filamin are present in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin, and myosin of actin filaments, provide dynamic character of the network.
Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or multiple shorter structures called cilia. These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin, and are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (mastigonemes), as in many stramenopiles. Their interior is continuous with the cell's cytoplasm.
Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division.
The cells of plants, algae, fungi and most chromalveolates, but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell.
The major polysaccharides making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.
Eukaryotes have a life cycle that involves sexual reproduction, alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa, to form a zygote; this may grow into a body, with its cells dividing by mitosis, and at some stage produce haploid gametes through meiosis, a division that reduces the number of chromosomes and creates genetic variability. There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases. Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have a smaller surface area to volume ratio.
The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. A core set of genes that function in meiosis is present in both *Trichomonas vaginalis* and *Giardia intestinalis*, two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as *Leishmania* parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.
In antiquity, the two lineages of animals and plants were recognized by Aristotle and Theophrastus. The lineages were given the taxonomic rank of kingdom by Linnaeus in the 18th century. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word *Protozoa* to refer to organisms such as ciliates, and this group was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista, in 1866. The eukaryotes thus came to be seen as four kingdoms:
The protists were at that time thought to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing, leading to a system of domains rather than kingdoms as top level rank being put forward by Carl Woese, Otto Kandler, and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that "'eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create a "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)".
By 2014, a rough consensus started to emerge from the phylogenomic studies of the previous two decades. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diphoda (formerly bikonts), which includes plants and most algal lineages. A third major grouping, the Excavata, has been abandoned as a formal group as it was found to be paraphyletic. The proposed phylogeny below includes two groups of excavates (Discoba and Metamonada), and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes. The Provora are a group of microbial predators discovered in 2022.
One view of the great kingdoms and their stem groups. The Metamonada are hard to place, being sister possibly to Discoba or to Malawimonada or being a paraphyletic group external to all other eukaryotes.
The origin of the eukaryotic cell, or *eukaryogenesis*, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The last eukaryotic common ancestor (LECA) is the hypothetical origin of all living eukaryotes, and was most likely a biological population, not a single individual. The LECA is believed to have been a protist with a nucleus, at least one centriole and flagellum, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin or cellulose, and peroxisomes.
An endosymbiotic union between a motile anaerobic archaean and an aerobic alphaproteobacterium gave rise to the LECA and all eukaryotes with mitochondria. A second, much later endosymbiosis with a cyanobacterium gave rise to the ancestor of plants, with chloroplasts.
The presence of eukaryotic biomarkers in archaea points towards an archaeal origin. The genomes of Promethearchaeati archaea have plenty of eukaryotic signature protein genes, which play a crucial role in the development of the cytoskeleton and complex cellular structures characteristic of eukaryotes. In 2022, cryo-electron tomography demonstrated that Promethearchaeati archaea have a complex actin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.
The timing of the origin of eukaryotes is hard to determine, but the discovery of *Qingshania magnificia*, the earliest multicelluar eukaryote from North China which lived 1.635 billion years ago, suggests that the crown group eukaryotes originated from the late Paleoproterozoic (Statherian). The earliest unequivocal unicellular eukaryotes, *Tappania plana*, *Shuiyousphaeridium macroreticulatum*, *Dictyosphaera macroreticulata*, *Germinosphaera alveolata*, and *Valeria lophostriata* from North China, lived approximately 1.65 billion years ago.
Some acritarchs are known from at least 1.65 billion years ago, and a fossil, *Grypania*, which may be an alga, is as much as 2.1 billion years old. The "problematic" fossil *Diskagma* has been found in paleosols 2.2 billion years old.
The Neoarchean fossil *Thuchomyces* shares similarities with eukaryotes, specifically fungi. It especially resembles the problematic fossil *Diskagma*, with hyphae and multiple differentiated layers. However, it is over 600 million years older than all other possible eukaryotes, and many of its "eukaryote features" are not specific to the clade, meaning it is almost certainly a microbial mat instead.
Structures proposed to represent "large colonial organisms" have been found in the black shales of the Palaeoproterozoic such as the Francevillian B Formation, in Gabon, dubbed the "Francevillian biota" which is dated at 2.1 billion years old. However, the status of these structures as fossils is contested, with other authors suggesting that they might represent pseudofossils. The oldest fossils that can unambiguously be assigned to eukaryotes are from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.
The presence of steranes, eukaryotic-specific biomarkers, in Australian shales previously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old, but these Archaean biomarkers have been rebutted as later contaminants. The oldest valid biomarker records are only around 800 million years old. In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago. The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria.
Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive increase in the zinc composition of marine sediments has been attributed to the rise of substantial populations of eukaryotes, which preferentially consume and incorporate zinc relative to prokaryotes, approximately a billion years after their origin (at the latest).
- Eukaryote hybrid genome
- List of sequenced eukaryotic genomes
*Parakaryon myojinensis*- Vault (organelle)
- "Eukaryotes" Archived 29 January 2012 at the Wayback Machine (Tree of Life Web Project)
- "
*Eukaryote*".*The Encyclopedia of Life*.
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# Bird - Wikipedia
| Birds Temporal range:
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| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
Clade:
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Sauropsida |
Clade:
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Archosauria |
Clade:
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Avemetatarsalia |
Clade:
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Dinosauria |
Clade:
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Theropoda |
Clade:
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Ornithurae |
| Class: | Aves Linnaeus, 1758 |
| Extant clades | |
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| Synonyms | |
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**Birds** are a group of warm-blooded vertebrates constituting the class **Aves** (Latin: [ˈaːwεs]), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are over 11,000 living species and they are split into 44 orders. More than half are passerine or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.
Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs. Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Birds are descendants of the primitive avialans (whose members include *Archaeopteryx*) which first appeared during the Late Jurassic. According to some estimates, modern birds (**Neornithes**) evolved in the Late Cretaceous or between the Early and Late Cretaceous (100 Ma) and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-ornithuran dinosaurs.
Many social species preserve knowledge across generations (culture). Birds are social, communicating with visual signals, calls, and songs, and participating in such behaviour as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous, usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.
Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.
## Evolution and classification
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume *Ornithologiae*.
Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the clade Theropoda as an infraclass or more recently a subclass or class.
### Definition
Aves and a sister group, the order Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and *Archaeopteryx lithographica*. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents to the *PhyloCode*. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the broader group Avialae, on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.
Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. They assigned other names to the other groups.
The birds' phylogenetic relationships to major living reptile groups. (The turtles' position is uncertain: Some authorities embed them inside the Archosaurs, with birds and crocodiles.)
|
- Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
- Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
- Aves can mean those feathered dinosaurs that fly (alternately Avialae)
- Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group", in this sense synonymous with
**Neornithes**)
Under the fourth definition *Archaeopteryx*, traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.
| Cladogram showing the results of a phylogenetic study by Cau, 2018. |
Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to *Deinonychus*", with *Troodon* being sometimes added as a second external specifier in case it is closer to birds than to *Deinonychus*. Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.
Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma.
### Dinosaurs and the origin of birds
Cladogram following the results of a phylogenetic study by Cau et al., 2015
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Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs and, more specifically, members of Maniraptora, a group of theropods which includes dromaeosaurids and oviraptorosaurs, among others. As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs, contributed to this ambiguity.
The consensus view in contemporary palaeontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these form a group called Paraves. Some basal members of Deinonychosauria, such as *Microraptor*, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Unlike *Archaeopteryx* and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that the first avialans were omnivores.
The Late Jurassic *Archaeopteryx* is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. *Archaeopteryx* was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.
### Early evolution
Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs, i.e. the first dinosaurs closer to living birds than to *Tyrannosaurus rex*. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. After the appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer. The integument evolved into complex, pennaceous feathers.
The oldest known paravian (and probably the earliest avialan) fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The avialan species from this time period include *Anchiornis huxleyi*, *Xiaotingia zhengi*, and *Aurornis xui*.
The well-known probable early avialan, *Archaeopteryx*, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.
Avialans diversified into a wide variety of forms during the Cretaceous period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle, an ossification of fused tail vertebrae. In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size. Around 95 million years ago, they evolved a better sense of smell.
A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula, and the loss of grasping hands.
Cladogram following the results of a phylogenetic study by Cau et al., 2015
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### Early diversity of bird ancestors
| Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis |
The first large, diverse lineage of short-tailed avialans to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, Enantiornithes became extinct along with many other dinosaur groups at the end of the Mesozoic era.
Many species of the second major avialan lineage to diversify, the Euornithes (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithians lacked perching adaptations and likely included shorebird-like species, waders, and swimming and diving species.
The latter included the superficially gull-like *Ichthyornis* and the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. The early euornithians also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithians retained teeth in other parts of the jaws). Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers, which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.
A study on mosaic evolution in the avian skull found that the last common ancestor of all Neornithes might have had a beak similar to that of the modern hook-billed vanga and a skull similar to that of the Eurasian golden oriole. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor.
### Diversification of modern birds
| Major groups of modern birds based on Sibley-Ahlquist taxonomy |
Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Early Cretaceous to the latest Cretaceous. Similarly, there is no agreement on whether most of the early diversification of modern birds occurred in the Cretaceous and associated with breakup of the supercontinent Gondwana or occurred later and potentially as a consequence of the Cretaceous–Palaeogene extinction event. This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous evolutionary radiation, while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy).
The discovery in 2005 of *Vegavis* from the Maastrichtian, the last stage of the Late Cretaceous, proved that the diversification of modern birds started before the Cenozoic era. The affinities of an earlier fossil, the possible galliform *Austinornis lentus*, dated to about 85 million years ago, are still too controversial to provide a fossil evidence of modern bird diversification. In 2020, *Asteriornis* from the Maastrichtian was described, it appears to be a close relative of Galloanserae, the earliest diverging lineage within Neognathae.
Attempts to reconcile molecular and fossil evidence using genomic-scale DNA data and comprehensive fossil information have not resolved the controversy. However, a 2015 estimate that used a new method for calibrating molecular clocks confirmed that while modern birds originated early in the Late Cretaceous, likely in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds would have expanded from West Gondwana through two routes. One route was an Antarctic interchange in the Paleogene. The other route was probably via Paleocene land bridges between South America and North America, which allowed for the rapid expansion and diversification of Neornithes into the Holarctic and Paleotropics. On the other hand, the occurrence of *Asteriornis* in the Northern Hemisphere suggest that Neornithes dispersed out of East Gondwana before the Paleocene.
### Classification of bird orders
All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds. These two subdivisions have variously been given the rank of superorder, cohort, or infraclass. The number of known living bird species is around 11,000 although sources may differ in their precise numbers.
Cladogram of modern bird relationships based on Stiller *et al* (2024)., showing the 44 orders recognised by the IOC.
Aves
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The classification of birds is a contentious issue. Sibley and Ahlquist's *Phylogeny and Classification of Birds* (1990) is a landmark work on the subject. Most evidence seems to suggest the assignment of orders is accurate, but scientists disagree about the relationships among the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. Fossil and molecular evidence from the 2010s is providing an increasingly clear picture of the evolution of modern bird orders.
### Genomics
In 2010, the genome had been sequenced for only two birds, the chicken and the zebra finch. As of 2022[update], the genomes of 542 species of birds had been completed. At least one genome has been sequenced from every order. These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the *Howard and Moore Checklist*).
Being able to sequence and compare whole genomes gives researchers many types of information, about genes, the DNA that regulates the genes, and their evolutionary history. This has led to reconsideration of some of the classifications that were based solely on the identification of protein-coding genes. Waterbirds such as pelicans and flamingos, for example, may have in common specific adaptations suited to their environment that were developed independently.
## Distribution
Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the snow petrel's breeding colonies up to 440 kilometres (270 mi) inland in Antarctica. The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics; however studies from the 2000s found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics. Many species migrate annually over great distances and across oceans; several families of birds have adapted to life both on the world's oceans and in them, and some seabird species come ashore only to breed, while some penguins have been recorded diving up to 300 metres (980 ft) deep.
Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the ring-necked pheasant, for example, has been introduced around the world as a game bird. Others have been accidental, such as the establishment of wild monk parakeets in several North American cities after their escape from captivity. Some species, including cattle egret, yellow-headed caracara and galah, have spread naturally far beyond their original ranges as agricultural expansion created alternative habitats although modern practices of intensive agriculture have negatively impacted farmland bird populations.
## Anatomy and physiology
Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.
### Skeletal system
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the respiratory system. The skull bones in adults are fused and do not show cranial sutures. The orbital cavities that house the eyeballs are large and separated from each other by a bony septum (partition). The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings. The wings are more or less developed depending on the species; the only known groups that lost their wings are the extinct moa and elephant birds.
### Excretory system
Like reptiles, birds are primarily uricotelic; that is, their kidneys extract nitrogenous waste from their bloodstream and excrete it as uric acid, instead of urea or ammonia, through the ureters into the intestine. Birds do not have a urinary bladder or external urethral opening. With the exception of the ostrich, uric acid is excreted along with faeces as a semisolid waste. However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. They also excrete creatine, rather than creatinine like mammals. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets.
It is a common but not universal feature of altricial passerine nestlings (born helpless, under constant parental care) that instead of excreting directly into the nest, they produce a fecal sac. This is a mucus-covered pouch that allows parents to either dispose of the waste outside the nest or to recycle the waste through their own digestive system.
### Reproductive system
Most male birds do not have intromittent penises. Males within Palaeognathae (with the exception of the kiwis), the Anseriformes (with the exception of screamers), and in rudimentary forms in Galliformes (but fully developed in Cracidae) possess a penis, which is never present in Neoaves. Its length is thought to be related to sperm competition and it fills with lymphatic fluid instead of blood when erect. When not copulating, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. Female birds have sperm storage tubules that allow sperm to remain viable long after copulation, a hundred days in some species. Sperm from multiple males may compete through this mechanism. Most female birds have a single ovary and a single oviduct, both on the left side, but there are exceptions: species in at least 16 different orders of birds have two ovaries. Even these species, however, tend to have a single oviduct. It has been speculated that this might be an adaptation to flight, but males have two testes, and it is also observed that the gonads in both sexes decrease dramatically in size outside the breeding season. Also terrestrial birds generally have a single ovary, as does the platypus, an egg-laying mammal. A more likely explanation is that the egg develops a shell while passing through the oviduct over a period of about a day, so that if two eggs were to develop at the same time, there would be a risk to survival. While rare, mostly abortive, parthenogenesis is not unknown in birds and eggs can be diploid, automictic and results in male offspring.
Birds are solely gonochoric, meaning they have two sexes: either female or male. The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ). A complex system of disassortative mating with two morphs is involved in the white-throated sparrow *Zonotrichia albicollis*, where white- and tan-browed morphs of opposite sex pair, making it appear as if four sexes were involved since any individual is compatible with only a fourth of the population.
In nearly all species of birds, an individual's sex is determined at fertilisation. However, one 2007 study claimed to demonstrate temperature-dependent sex determination among the Australian brushturkey, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio. This, however, was later proven to not be the case. These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality.
### Respiratory and circulatory systems
Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Sound production is achieved using the syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea; the trachea being elongated in some species, increasing the volume of vocalisations and the perception of the bird's size.
In birds, the main arteries taking blood away from the heart originate from the right aortic arch (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the aorta. The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating red blood cells in birds retain their nucleus.
#### Heart type and features
The avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac. This pericardial sac is filled with a serous fluid for lubrication. The heart itself is divided into a right and left half, each with an atrium and ventricle. The atrium and ventricles of each side are separated by atrioventricular valves which prevent back flow from one chamber to the next during contraction. Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium.
The sinoatrial node uses calcium to cause a depolarising signal transduction pathway from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. The avian heart also consists of muscular arches that are made up of thick bundles of muscular layers. Much like a mammalian heart, the avian heart is composed of endocardial, myocardial and epicardial layers. The atrium walls tend to be thinner than the ventricle walls, due to the intense ventricular contraction used to pump oxygenated blood throughout the body. Avian hearts are generally larger than mammalian hearts when compared to body mass. This adaptation allows more blood to be pumped to meet the high metabolic need associated with flight.
#### Organisation
Birds have a very efficient system for diffusing oxygen into the blood; birds have a ten times greater surface area to gas exchange volume than mammals. As a result, birds have more blood in their capillaries per unit of volume of lung than a mammal. The arteries are composed of thick elastic muscles to withstand the pressure of the ventricular contractions, and become more rigid as they move away from the heart. Blood moves through the arteries, which undergo vasoconstriction, and into arterioles which act as a transportation system to distribute primarily oxygen as well as nutrients to all tissues of the body. As the arterioles move away from the heart and into individual organs and tissues they are further divided to increase surface area and slow blood flow. Blood travels through the arterioles and moves into the capillaries where gas exchange can occur.
Capillaries are organised into capillary beds in tissues; it is here that blood exchanges oxygen for carbon dioxide waste. In the capillary beds, blood flow is slowed to allow maximum diffusion of oxygen into the tissues. Once the blood has become deoxygenated, it travels through venules then veins and back to the heart. Veins, unlike arteries, are thin and rigid as they do not need to withstand extreme pressure. As blood travels through the venules to the veins a funneling occurs called vasodilation bringing blood back to the heart. Once the blood reaches the heart, it moves first into the right atrium, then the right ventricle to be pumped through the lungs for further gas exchange of carbon dioxide waste for oxygen. Oxygenated blood then flows from the lungs through the left atrium to the left ventricle where it is pumped out to the body.
### Nervous system
The nervous system is large relative to the bird's size. The most developed part of the brain of birds is the one that controls the flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. Most birds have a poor sense of smell with notable exceptions including kiwis, New World vultures and tubenoses. The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Some species also have dual fovea. Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones. They also have double cones, likely to mediate achromatic vision.
Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Male blue tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers. Ultraviolet light is also used in foraging—kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents. With the exception of pigeons and a few other species, the eyelids of birds are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds. The bird retina has a fan shaped blood supply system called the pecten.
Eyes of most birds are large, not very round and capable of only limited movement in the orbits, typically 10–20°. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field. The avian ear lacks external pinnae but is covered by feathers, although in some birds, such as the *Asio*, *Bubo* and *Otus* owls, these feathers form tufts which resemble ears. The inner ear has a cochlea, but it is not a spiral as in mammals. Several species have been demonstrated to hear infrasound (below 20 Hz) and a few cave-dwelling swifts and oilbirds emit ultrasound (above 20 kHz) and echolocate in darkness.
### Defence and intraspecific combat
A few species are able to use chemical defences against predators; some Procellariiformes can eject an unpleasant stomach oil against an aggressor, and some species of pitohuis from New Guinea have a powerful neurotoxin in their skin and feathers.
A lack of field observations limit our knowledge, but intraspecific conflicts are known to sometimes result in injury or death. The screamers (Anhimidae), some jacanas (*Jacana*, *Hydrophasianus*), the spur-winged goose (*Plectropterus*), the torrent duck (*Merganetta*) and nine species of lapwing (*Vanellus*) use a sharp spur on the wing as a weapon. The steamer ducks (*Tachyeres*), geese and swans (*Anserinae*), the solitaire (*Pezophaps*), sheathbills (*Chionis*), some guans (*Crax*) and stone curlews (*Burhinus*) use a bony knob on the alular metacarpal to punch and hammer opponents. The jacanas *Actophilornis* and *Irediparra* have an expanded, blade-like radius. The extinct *Xenicibis* was unique in having an elongate forelimb and massive hand which likely functioned in combat or defence as a jointed club or flail. Swans, for instance, may strike with the bony spurs and bite when defending eggs or young.
Feathers are a feature characteristic of birds (though also present in some dinosaurs not currently considered to be true birds). They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signalling. There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex.
Plumage is regularly moulted; the standard plumage of a bird that has moulted after breeding is known as the "" plumage, or—in the Humphrey–Parkes terminology—"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey–Parkes system as "" plumages. Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, flight feathers are replaced one at a time with the innermost being the first. When the fifth of sixth primary is replaced, the outermost begin to drop. After the innermost tertiaries are moulted, the starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary are moulted in synchrony with the primary that they overlap.
A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. As a general rule, the tail feathers are moulted and replaced starting with the innermost pair. Centripetal moults of tail feathers are however seen in the Phasianidae. The centrifugal moult is modified in the tail feathers of woodpeckers and treecreepers, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail. The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from centre outward. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation.
Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland; these secretions protect the feathers' flexibility and act as an antimicrobial agent, inhibiting the growth of feather-degrading bacteria. This may be supplemented with the secretions of formic acid from ants, which birds receive through a behaviour known as anting, to remove feather parasites.
The scales of birds are composed of the same keratin as beaks, claws, and spurs. They are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers. The scales of birds are thought to be homologous to those of reptiles and mammals.
### Flight
Most birds can fly, which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb (wing) that serves as an aerofoil.
Wing shape and size generally determine a bird's flight style and performance; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are flightless, as were many extinct birds. Flightlessness often arises in birds on isolated islands, most likely due to limited resources and the absence of mammalian land predators. Flightlessness is almost exclusively correlated with gigantism due to an island's inherent condition of isolation. Although flightless, penguins use similar musculature and movements to "fly" through the water, as do some flight-capable birds such as auks, shearwaters and dippers.
## Behaviour
Most birds are diurnal, but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.
### Diet and feeding
are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds. The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth. Some species such as pigeons and some psittacine species do not have a gallbladder. Most birds are highly adapted for rapid digestion to aid with flight. Some migratory birds have adapted to use protein stored in many parts of their bodies, including protein from the intestines, as additional energy during migration.
Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists. Avian foraging strategies can vary widely by species. Many birds glean for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek pest insects are considered beneficial 'biological control agents' and their presence encouraged in biological pest control programmes. Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually.
Nectar feeders such as hummingbirds, sunbirds, lories, and lorikeets amongst others have specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers. Kiwis and shorebirds with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of ecological niches. Divers, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. Flamingos, three species of prion, and some ducks are filter feeders. Geese and dabbling ducks are primarily grazers.
Some species, including frigatebirds, gulls, and skuas, engage in kleptoparasitism, stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet; a study of great frigatebirds stealing from masked boobies estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%. Other birds are scavengers; some of these, like vultures, are specialised carrion eaters, while others, like gulls, corvids, or other birds of prey, are opportunists.
### Water and drinking
Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands. Some desert birds can obtain their water needs entirely from moisture in their food. Some have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting. Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.
Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the pigeon, finch, mousebird, button-quail and bustard families are capable of sucking up water without the need to tilt back their heads. Some desert birds depend on water sources and sandgrouse are particularly well known for congregating daily at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers. Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called crop milk that they provide to their chicks.
### Feather care
Feathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, ectoparasitic feather mites and bird lice. The physical condition of feathers are maintained by often with the application of secretions from the . Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as anting in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.
### Migration
Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, shorebirds, and waterbirds undertake annual long-distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the temperate or polar regions and a non-breeding season in the tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.
Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi), although the bar-tailed godwit is capable of non-stop flights of up to 10,200 km (6,300 mi). Some seabirds undertake long migrations, with the longest annual migrations including those of Arctic terns, which were recorded travelling an average of 70,900 km (44,100 mi) between their Arctic breeding grounds in Greenland and Iceland and their wintering grounds in Antarctica, with one bird covering 81,600 km (50,700 mi), and sooty shearwaters, which nest in New Zealand and Chile and make annual round trips of 64,000 km (39,800 mi) to their summer feeding grounds in the North Pacific off Japan, Alaska and California. Other seabirds disperse after breeding, travelling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability. Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates. Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.
Altitudinal migration is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories also become inhospitable due to lack of food. Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.
The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s, a Manx shearwater released in Boston in the United States returned to its colony in Skomer, in Wales within 13 days, a distance of 5,150 km (3,200 mi). Birds navigate during migration using a variety of methods. For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an internal clock. Orientation with the stellar compass depends on the position of the constellations surrounding Polaris. These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors.
### Communication
Birds communicate primarily using visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).
Birds sometimes use plumage to assess and assert social dominance, to display breeding condition in sexually selected species, or to make threatening displays, as in the sunbittern's mimicry of a large predator to ward off hawks and protect young chicks.
Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds. The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements; males' breeding success may depend on the quality of such displays.
Bird calls and songs, which are produced in the syrinx, are the major means by which birds communicate with sound. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs. Calls are used for a variety of purposes, including mate attraction, evaluation of potential mates, bond formation, the claiming and maintenance of territories, the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season), and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat. Some birds also use mechanical sounds for auditory communication. The *Coenocorypha* snipes of New Zealand drive air through their feathers, woodpeckers drum for long-distance communication, and palm cockatoos use tools to drum.
### Flocking and other associations
While some birds are essentially territorial or live in small family groups, other birds may form large flocks. The principal benefits of flocking are safety in numbers and increased foraging efficiency. Defence against predators is particularly important in closed habitats like forests, where ambush predation is common and multiple eyes can provide a valuable early warning system. This has led to the development of many mixed-species feeding flocks, which are usually composed of small numbers of many species; these flocks provide safety in numbers but increase potential competition for resources. Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases. Some species have a mixed system with breeding pairs maintaining territories, while unmated or young birds live in flocks where they secure mates prior to finding territories.
Birds sometimes also form associations with non-avian species. Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface. Some species of hornbills have a mutualistic relationship with dwarf mongooses, in which they forage together and warn each other of nearby birds of prey and other predators.
### Resting and roosting
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats. Swifts are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight. It has been suggested that there may be certain kinds of sleep which are possible even when in flight.
Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by viewing the outer margins of the flock. This adaptation is also known from marine mammals. Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators. Roosting sites are often chosen with regard to thermoregulation and safety. Unusual mobile roost sites include large herbivores on the African savanna that are used by oxpeckers.
Many sleeping birds bend their heads over their backs and tuck their bills in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. Perching birds have a tendon-locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus *Loriculus* roost hanging upside down. Some hummingbirds go into a nightly state of torpor accompanied with a reduction of their metabolic rates. This physiological adaptation shows in nearly a hundred other species, including owlet-nightjars, nightjars, and woodswallows. One species, the common poorwill, even enters a state of hibernation. Birds do not have sweat glands, but can lose water directly through the skin, and they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or using special behaviours like urohidrosis to cool themselves.
### Breeding
#### Social systems
95 per cent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate. Monogamy allows for both paternal care and biparental care, which is especially important for species in which care from both the female and the male parent is required in order to successfully rear a brood. Among many socially monogamous species, extra-pair copulation (infidelity) is common. Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of forced copulation in ducks and other anatids.
For females, possible benefits of extra-pair copulation include getting better genes for her offspring and insuring against the possibility of infertility in her mate. Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.
Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur. Polygamous breeding systems arise when females are able to raise broods without the help of males. Mating systems vary across bird families but variations within species are thought to be driven by environmental conditions. A unique system is the formation of trios where a third individual is allowed by a breeding pair temporarily into the territory to assist with brood raising thereby leading to higher fitness.
Breeding usually involves some form of courtship display, typically performed by the male. Most displays are rather simple and involve some type of song. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Females are generally the ones that drive partner selection, although in the polyandrous phalaropes, this is reversed: plainer males choose brightly coloured females. Courtship feeding, billing and are commonly performed between partners, generally after the birds have paired and mated.
Homosexual behaviour has been observed in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks. Over 130 avian species around the world engage in sexual interactions between the same sex or homosexual behaviours. "Same-sex courtship activities may involve elaborate displays, synchronised dances, gift-giving ceremonies, or behaviours at specific display areas including bowers, arenas, or leks."
Many birds actively defend a territory from others of the same species during the breeding season; maintenance of territories protects the food source for their chicks. Species that are unable to defend feeding territories, such as seabirds and swifts, often breed in colonies instead; this is thought to offer protection from predators. Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense.
All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay camouflaged eggs. There are many exceptions to this pattern, however; the ground-nesting nightjars have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of brood parasites have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts.
Bird eggs are usually laid in a nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, mounds, or burrows. Some bird nests can be a simple scrape, with minimal or no lining; most seabird and wader nests are no more than a scrape on the ground. Most birds build nests in sheltered, hidden areas to avoid predation, but large or colonial birds—which are more capable of defence—may build more open nests. During nest construction, some species seek out plant matter from plants with parasite-reducing toxins to improve chick survival, and feathers are often used for nest insulation. Some bird species have no nests; the cliff-nesting common guillemot lays its eggs on bare rock, and male emperor penguins keep eggs between their body and feet. The absence of nests is especially prevalent in open habitat ground-nesting species where any addition of nest material would make the nest more conspicuous. Many ground nesting birds lay a clutch of eggs that hatch synchronously, with precocial chicks led away from the nests (nidifugous) by their parents soon after hatching.
Incubation, which regulates temperature for chick development, usually begins after the last egg has been laid. In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Warmth from parents passes to the eggs through brood patches, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process; adult albatrosses, for instance, lose as much as 83 grams (2.9 oz) of body weight per day of incubation. The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or volcanic sources. Incubation periods range from 10 days (in woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).
The diversity of characteristics of birds is great, sometimes even in closely related species. Several avian characteristics are compared in the table below.
| Species | Adult weight (grams) |
Incubation (days) |
Clutches (per year) |
Clutch size |
|---|---|---|---|---|
Ruby-throated hummingbird (Archilochus colubris)
|
3 | 13 | 2.0 | 2 |
House sparrow (Passer domesticus)
|
25 | 11 | 4.5 | 5 |
Greater roadrunner (Geococcyx californianus)
|
376 | 20 | 1.5 | 4 |
Turkey vulture (Cathartes aura)
|
2,200 | 39 | 1.0 | 2 |
Laysan albatross (Phoebastria immutabilis)
|
3,150 | 64 | 1.0 | 1 |
Magellanic penguin (Spheniscus magellanicus)
|
4,000 | 40 | 1.0 | 1 |
Golden eagle (Aquila chrysaetos)
|
4,800 | 40 | 1.0 | 2 |
Wild turkey (Meleagris gallopavo)
|
6,050 | 28 | 1.0 | 11 |
#### Parental care and fledging
At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed *altricial*, and tend to be born small, blind, immobile and naked; chicks that are mobile and feathered upon hatching are termed *precocial*. Altricial chicks need help thermoregulating and must be brooded for longer than precocial chicks. The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on an "altricial-precocial spectrum". Chicks at neither extreme but favouring one or the other may be termed or .
The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in megapodes ends at hatching; the newly hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately. At the other extreme, many seabirds have extended periods of parental care, the longest being that of the great frigatebird, whose chicks take up to six months to fledge and are fed by the parents for up to an additional 14 months. The *chick guard stage* describes the period of breeding during which one of the adult birds is permanently present at the nest after chicks have hatched. The main purpose of the guard stage is to aid offspring to thermoregulate and protect them from predation.
In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of only one sex. In some species, other members of the same species—usually close relatives of the breeding pair, such as offspring from previous broods—will help with the raising of the young. Such alloparenting is particularly common among the Corvida, which includes such birds as the true crows, Australian magpie and fairy-wrens, but has been observed in species as different as the rifleman and red kite. Among most groups of animals, male parental care is rare. In birds, however, it is quite common—more so than in any other vertebrate class. Although territory and nest site defence, incubation, and chick feeding are often shared tasks, there is sometimes a division of labour in which one mate undertakes all or most of a particular duty.
The point at which chicks fledge varies dramatically. The chicks of the *Synthliboramphus* murrelets, like the ancient murrelet, leave the nest the night after they hatch, following their parents out to sea, where they are raised away from terrestrial predators. Some other species, such as ducks, move their chicks away from the nest at an early age. In most species, chicks leave the nest just before, or soon after, they are able to fly. The amount of parental care after fledging varies; albatross chicks leave the nest on their own and receive no further help, while other species continue some supplementary feeding after fledging. Chicks may also follow their parents during their first migration.
#### Brood parasites
Brood parasitism, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism. After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either *obligate brood parasites*, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or *non-obligate brood parasites*, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young. One hundred bird species, including honeyguides, icterids, and ducks, are obligate parasites, though the most famous are the cuckoos. Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks; this ensures that all food brought to the nest will be fed to the parasitic chicks.
#### Sexual selection
Birds have evolved a variety of mating behaviours, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and colour differences are energetically costly attributes that signal competitive breeding situations. Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signalling adaptations ensure that sexually selected ornaments such as plumage colouration and courtship behaviour are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours.
#### Inbreeding depression
Inbreeding causes early death (inbreeding depression) in the zebra finch *Taeniopygia guttata*. Embryo survival (that is, hatching success of fertile eggs) was significantly lower for sib-sib mating pairs than for unrelated pairs.
Darwin's finch *Geospiza scandens* experiences inbreeding depression (reduced survival of offspring) and the magnitude of this effect is influenced by environmental conditions such as low food availability.
#### Inbreeding avoidance
Incestuous matings by the purple-crowned fairy wren *Malurus coronatus* result in severe fitness costs due to inbreeding depression (greater than 30% reduction in hatchability of eggs). Females paired with related males may undertake extra pair matings (see Promiscuity#Other animals for 90% frequency in avian species) that can reduce the negative effects of inbreeding. However, there are ecological and demographic constraints on extra pair matings. Nevertheless, 43% of broods produced by incestuously paired females contained extra pair young.
Inbreeding depression occurs in the great tit (*Parus major*) when the offspring produced as a result of a mating between close relatives show reduced fitness. In natural populations of *Parus major*, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.
Southern pied babblers *Turdoides bicolor* appear to avoid inbreeding in two ways. The first is through dispersal, and the second is by avoiding familiar group members as mates.
Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin. Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups. In general, inbreeding is avoided because it leads to a reduction in progeny fitness (inbreeding depression) due largely to the homozygous expression of deleterious recessive alleles. Cross-fertilisation between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.
## Ecology
Birds occupy a wide range of ecological positions. While some birds are generalists, others are highly specialised in their habitat or food requirements. Even within a single habitat, such as a forest, the niches occupied by different species of birds vary, with some species feeding in the forest canopy, others beneath the canopy, and still others on the forest floor. Forest birds may be insectivores, frugivores, or nectarivores. Aquatic birds generally feed by fishing, plant eating, and piracy or kleptoparasitism. Many grassland birds are granivores. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised scavengers. Birds are also preyed upon by a range of mammals including a few avivorous bats. A wide range of endo- and ectoparasites depend on birds and some parasites that are transmitted from parent to young have co-evolved and show host-specificity.
Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal. Plants and pollinating birds often coevolve, and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.
Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfil ecological roles typically played by larger animals. For example, in New Zealand nine species of moa were important browsers, as are the kererū and kōkako today. Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.
Many birds act as ecosystem engineers through the construction of nests, which provide important microhabitats and food for hundreds of species of invertebrates. Nesting seabirds may affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of guano, which may enrich the local soil and the surrounding seas.
A wide variety of avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.
## Relationship with humans
Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man. Sometimes, these relationships are mutualistic, like the cooperative honey-gathering among honeyguides and African peoples such as the Borana. Other times, they may be commensal, as when species such as the house sparrow have benefited from human activities. Several species have reconciled to habits of farmers who practice traditional farming. Examples include the Sarus Crane that begins nesting in India when farmers flood the fields in anticipation of rains, and the woolly-necked storks that have taken to nesting on a short tree grown for agroforestry beside fields and canals. Several bird species have become commercially significant agricultural pests, and some pose an aviation hazard. Human activities can also be detrimental, and have threatened numerous bird species with extinction (hunting, avian lead poisoning, pesticides, roadkill, wind turbine kills and predation by pet cats and dogs are common causes of death for birds).
Birds can act as vectors for spreading diseases such as psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis, and cryptosporidiosis over long distances. Some of these are zoonotic diseases that can also be transmitted to humans.
### Economic importance
Domesticated birds raised for meat and eggs, called poultry, are the largest source of animal protein eaten by humans; in 2003, 76 million tons of poultry and 61 million tons of eggs were produced worldwide. Chickens account for much of human poultry consumption, though domesticated turkeys, ducks, and geese are also relatively common. Many species of birds are also hunted for meat. Bird hunting is primarily a recreational activity except in extremely undeveloped areas. The most important birds hunted in North and South America are waterfowl; other widely hunted birds include pheasants, wild turkeys, quail, doves, partridge, grouse, snipe, and woodcock. Muttonbirding is also popular in Australia and New Zealand. Although some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species.
Other commercially valuable products from birds include feathers (especially the down of geese and ducks), which are used as insulation in clothing and bedding, and seabird faeces (guano), which is a valuable source of phosphorus and nitrogen. The War of the Pacific, sometimes called the Guano War, was fought in part over the control of guano deposits.
Birds have been domesticated by humans both as pets and for practical purposes. Colourful birds, such as parrots and mynas, are bred in captivity or kept as pets, a practice that has led to the illegal trafficking of some endangered species. Falcons and cormorants have long been used for hunting and fishing, respectively. Messenger pigeons, used since at least 1 AD, remained important as recently as World War II. Today, such activities are more common either as hobbies, for entertainment and tourism.
Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions. Many homeowners erect bird feeders near their homes to attract various species. Bird feeding has grown into a multimillion-dollar industry; for example, an estimated 75% of households in Britain provide food for birds at some point during the winter.
### In religion and mythology
Birds play prominent and diverse roles in religion and mythology. In religion, birds may serve as either messengers or priests and leaders for a deity, such as in the Cult of Makemake, in which the Tangata manu of Easter Island served as chiefs or as attendants, as in the case of Hugin and Munin, the two common ravens who whispered news into the ears of the Norse god Odin. In several civilisations of ancient Italy, particularly Etruscan and Roman religion, priests were involved in augury, or interpreting the words of birds while the "auspex" (from which the word "auspicious" is derived) watched their activities to foretell events.
They may also serve as religious symbols, as when Jonah (Hebrew: יונה, dove) embodied the fright, passivity, mourning, and beauty traditionally associated with doves. Birds have themselves been deified, as in the case of the common peacock, which is perceived as Mother Earth by the people of southern India. In the ancient world, doves were used as symbols of the Mesopotamian goddess Inanna (later known as Ishtar), the Canaanite mother goddess Asherah, and the Greek goddess Aphrodite. In ancient Greece, Athena, the goddess of wisdom and patron deity of the city of Athens, had a little owl as her symbol. In religious images preserved from the Inca and Tiwanaku empires, birds are depicted in the process of transgressing boundaries between earthly and underground spiritual realms. Indigenous peoples of the central Andes maintain legends of birds passing to and from metaphysical worlds.
### In culture and folklore
Birds have featured in culture and art since prehistoric times, when they were represented in early cave painting and carvings. Some birds have been perceived as monsters, including the mythological Roc and the Māori's legendary *Pouākai*, a giant bird capable of snatching humans. Birds were later used as symbols of power, as in the magnificent Peacock Throne of the Mughal and Persian emperors. With the advent of scientific interest in birds, many paintings of birds were commissioned for books.
Among the most famous of these bird artists was John James Audubon, whose paintings of North American birds were a great commercial success in Europe and who later lent his name to the National Audubon Society. Birds are also important figures in poetry; for example, Homer incorporated nightingales into his *Odyssey*, and Catullus used a sparrow as an erotic symbol in his Catullus 2. The relationship between an albatross and a sailor is the central theme of Samuel Taylor Coleridge's *The Rime of the Ancient Mariner*, which led to the use of the term as a metaphor for a 'burden'. Other English metaphors derive from birds; vulture funds and vulture investors, for instance, take their name from the scavenging vulture. Aircraft, particularly military aircraft, are frequently named after birds. The predatory nature of raptors make them popular choices for fighter aircraft such as the F-16 Fighting Falcon and the Harrier Jump Jet, while the names of seabirds may be chosen for aircraft primarily used by naval forces such as the HU-16 Albatross and the V-22 Osprey.
Perceptions of bird species vary across cultures. Owls are associated with bad luck, witchcraft, and death in parts of Africa, but are regarded as wise across much of Europe. Hoopoes were considered sacred in Ancient Egypt and symbols of virtue in Persia, but were thought of as thieves across much of Europe and harbingers of war in Scandinavia. In heraldry, birds, especially eagles, often appear in coats of arms In vexillology, birds are a popular choice on flags. Birds feature in the flag designs of 17 countries and numerous subnational entities and territories. Birds are used by nations to symbolise a country's identity and heritage, with 91 countries officially recognising a national bird. Birds of prey are highly represented, though some nations have chosen other species of birds with parrots being popular among smaller, tropical nations.
### In music
In music, birdsong has influenced composers and musicians in several ways: they can be inspired by birdsong; they can intentionally imitate bird song in a composition, as Vivaldi, Messiaen, and Beethoven did, along with many later composers; they can incorporate recordings of birds into their works, as Ottorino Respighi first did; or like Beatrice Harrison and David Rothenberg, they can duet with birds.
A 2023 archaeological excavation of a 10,000-year-old site in Israel yielded hollow wing bones of coots and ducks with perforations made on the side that are thought to have allowed them to be used as flutes or whistles possibly used by Natufian people to lure birds of prey.
## Threats and conservation
Human activities have caused population decreases or extinction in many bird species. Over a hundred bird species have gone extinct in historical times, although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of Melanesian, Polynesian, and Micronesian islands. Many bird populations are declining worldwide, with 1,227 species listed as threatened by BirdLife International and the IUCN in 2009. There have been long-term declines in North American bird populations, with an estimated loss of 2.9 billion breeding adults, about 30% of the total, since 1970.
The most commonly cited human threat to birds is habitat loss. Other threats include overhunting, accidental mortality due to collisions with buildings or vehicles, long-line fishing bycatch, pollution (including oil spills and pesticide use), competition and predation from nonnative invasive species, and climate change.
Governments and conservation groups work to protect birds, either by passing laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the California condor and Norfolk parakeet.
Human activities have allowed the expansion of a few temperate area species, such as the barn swallow and European starling. In the tropics and sub-tropics, relatively more species are expanding due to human activities, particularly due to the spread of crops such as rice whose expansion in south Asia has benefitted at least 64 bird species, though may have harmed many more species.
## See also
- Biodiversity loss
- Climate change and birds
- Glossary of bird terms
- List of individual birds
- Ornithology
- List of bird genera
- Breeding behaviors of birds
## Further reading
*All the Birds of the World*, Lynx Edicions, 2020.- Del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi (eds.).
*Handbook of the Birds of the World*(17-volume encyclopaedia), Lynx Edicions, Barcelona, 1992–2010. (*Vol. 1: Ostrich to Ducks*: ISBN 978-84-87334-10-8, etc.). - Lederer, Roger; Carol Burr (2014).
*Latein für Vogelbeobachter: über 3000 ornithologische Begriffe erklärt und erforscht*, aus dem Englischen übersetzt von Susanne Kuhlmannn-Krieg, Verlag DuMont, Köln, ISBN 978-3-8321-9491-8. *National Geographic Field Guide to Birds of North America*, National Geographic, 7th edition, 2017. ISBN 9781426218354*National Audubon Society Field Guide to North American Birds: Eastern Region*, National Audubon Society, Knopf.*National Audubon Society Field Guide to North American Birds: Western Region*, National Audubon Society, Knopf.- Svensson, Lars (2010).
*Birds of Europe*, Princeton University Press, second edition. ISBN 9780691143927 - Svensson, Lars (2010).
*Collins Bird Guide: The Most Complete Guide to the Birds of Britain and Europe*, Collins, 2nd edition. ISBN 978-0007268146
## External links
- Birdlife International – Dedicated to bird conservation worldwide; has a database with about 250,000 records on endangered bird species.
- Bird biogeography
- Birds and Science from the National Audubon Society
- Cornell Lab of Ornithology
- "
*Bird*".*The Encyclopedia of Life*. - Essays on bird biology
- North American Birds for Kids Archived 9 August 2010 at the Wayback Machine
- Ornithology
- Sora – Searchable online research archive; Archives of the following ornithological journals
*The Auk*,*Condor*,*Journal of Field Ornithology'*,*North American Bird Bander*,*Studies in Avian Biology*,*Pacific Coast Avifauna*, and the*Wilson Bulletin*. - The Internet Bird Collection – A free library of videos of the world's birds
- The Institute for Bird Populations, California
- List of field guides to birds, from the International Field Guides database
- RSPB bird identifier Archived 5 November 2013 at the Wayback Machine – Interactive identification of all UK birds
- Are Birds Really Dinosaurs? — University of California Museum of Paleontology.
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# List of states and territories of the United States - Wikipedia
The United States of America is a federal republic consisting of 50 states, a federal district (Washington, D.C., the capital city of the United States), five major territories, and various minor islands. Both the states and the United States as a whole are each sovereign jurisdictions. The Tenth Amendment to the United States Constitution allows states to exercise all powers of government not delegated to the federal government. Each state has its own constitution and government, and all states and their residents are represented in the federal Congress, a bicameral legislature consisting of the Senate and the House of Representatives. Each state elects two senators, while representatives are distributed among the states in proportion to the most recent constitutionally mandated decennial census. Additionally, each state is entitled to select a number of electors to vote in the Electoral College, the body that elects the president of the United States, equal to the total of representatives and senators in Congress from that state. The federal district does not have representatives in the Senate, but has a non-voting delegate in the House, and it is also entitled to electors in the Electoral College. Congress can admit more states, but it cannot create a new state from territory of an existing state or merge two or more states into one without the consent of all states involved, and each new state is admitted on an equal footing with the existing states.
The United States has control over fourteen territories. Five of them (American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the United States Virgin Islands) have a permanent, non-military population, while nine of them (the United States Minor Outlying Islands) do not. With the exception of Navassa Island, Puerto Rico, and the U.S. Virgin Islands, which are located in the Caribbean, all territories are located in the Pacific Ocean. One territory, Palmyra Atoll, is considered to be incorporated, meaning the full body of the Constitution has been applied to it; the other territories are unincorporated, meaning the Constitution does not fully apply to them. Ten territories (the Minor Outlying Islands and American Samoa) are considered to be unorganized, meaning they have not had an organic act enacted by Congress; the four other territories are organized, meaning an organic act has been enacted by Congress. The five inhabited territories each have limited autonomy in addition to having territorial legislatures and governors, but residents cannot vote in federal elections, although all are represented by non-voting delegates in the House.
The largest state by population is California, with a population of 39,538,223 people, while the smallest is Wyoming, with a population of 576,851 people; the federal district has a larger population (689,545) than both Wyoming and Vermont. The largest state by area is Alaska, encompassing 665,384 square miles (1,723,340 km2), while the smallest is Rhode Island, encompassing 1,545 square miles (4,000 km2). The most recent states to be admitted, Alaska and Hawaii, were admitted in 1959. The largest territory by population is Puerto Rico, with a population of 3,285,874 people (larger than 21 states), while the smallest is the Northern Mariana Islands, with a population of 47,329 people. Puerto Rico is the largest territory by area, encompassing 5,325 square miles (13,790 km2); the smallest territory, Kingman Reef, encompasses only 0.005 square miles (0.013 km2), or a little larger than 3 acres.
## States
| Flag, name and postal abbreviation |
Cities | Ratification or admission |
Population (2020) |
Total area | Reps. | |||
|---|---|---|---|---|---|---|---|---|
| Capital | Largest | mi2 |
km2
| |||||
| Alabama | AL | Montgomery | Huntsville | Dec 14, 1819 | 5,024,279 |
52,420 | 135,767 | 7
|
| Alaska | AK | Juneau | Anchorage | Jan 3, 1959 | 733,391 |
665,384 | 1,723,337 | 1
|
| Arizona | AZ | Phoenix | Feb 14, 1912 | 7,151,502 |
113,990 | 295,234 | 9
| |
| Arkansas | AR | Little Rock | Jun 15, 1836 | 3,011,524 |
53,179 | 137,732 | 4
| |
| California | CA | Sacramento | Los Angeles | Sep 9, 1850 | 39,538,223 |
163,695 | 423,967 | 52
|
| Colorado | CO | Denver | Aug 1, 1876 | 5,773,714 |
104,094 | 269,601 | 8
| |
| Connecticut | CT | Hartford | Bridgeport | Jan 9, 1788 | 3,605,944 |
5,543 | 14,357 | 5
|
| Delaware | DE | Dover | Wilmington | Dec 7, 1787 | 989,948 |
2,489 | 6,446 | 1
|
| Florida | FL | Tallahassee | Jacksonville | Mar 3, 1845 | 21,538,187 |
65,758 | 170,312 | 28
|
| Georgia | GA | Atlanta | Jan 2, 1788 | 10,711,908 |
59,425 | 153,910 | 14
| |
| Hawaii | HI | Honolulu | Aug 21, 1959 | 1,455,271 |
10,932 | 28,313 | 2
| |
| Idaho | ID | Boise | Jul 3, 1890 | 1,839,106 |
83,569 | 216,443 | 2
| |
| Illinois | IL | Springfield | Chicago | Dec 3, 1818 | 12,812,508 |
57,914 | 149,995 | 17
|
| Indiana | IN | Indianapolis | Dec 11, 1816 | 6,785,528 |
36,420 | 94,326 | 9
| |
| Iowa | IA | Des Moines | Dec 28, 1846 | 3,190,369 |
56,273 | 145,746 | 4
| |
| Kansas | KS | Topeka | Wichita | Jan 29, 1861 | 2,937,880 |
82,278 | 213,100 | 4
|
| Kentucky | KY | Frankfort | Louisville | Jun 1, 1792 | 4,505,836 |
40,408 | 104,656 | 6
|
| Louisiana | LA | Baton Rouge | New Orleans | Apr 30, 1812 | 4,657,757 |
52,378 | 135,659 | 6
|
| Maine | ME | Augusta | Portland | Mar 15, 1820 | 1,362,359 |
35,380 | 91,633 | 2
|
| Maryland | MD | Annapolis | Baltimore | Apr 28, 1788 | 6,177,224 |
12,406 | 32,131 | 8
|
| Massachusetts | MA | Boston | Feb 6, 1788 | 7,029,917 |
10,554 | 27,336 | 9
| |
| Michigan | MI | Lansing | Detroit | Jan 26, 1837 | 10,077,331 |
96,714 | 250,487 | 13
|
| Minnesota | MN | Saint Paul | Minneapolis | May 11, 1858 | 5,706,494 |
86,936 | 225,163 | 8
|
| Mississippi | MS | Jackson | Dec 10, 1817 | 2,961,279 |
48,432 | 125,438 | 4
| |
| Missouri | MO | Jefferson City | Kansas City | Aug 10, 1821 | 6,154,913 |
69,707 | 180,540 | 8
|
| Montana | MT | Helena | Billings | Nov 8, 1889 | 1,084,225 |
147,040 | 380,831 | 2
|
| Nebraska | NE | Lincoln | Omaha | Mar 1, 1867 | 1,961,504 |
77,348 | 200,330 | 3
|
| Nevada | NV | Carson City | Las Vegas | Oct 31, 1864 | 3,104,614 |
110,572 | 286,380 | 4
|
| New Hampshire | NH | Concord | Manchester | Jun 21, 1788 | 1,377,529 |
9,349 | 24,214 | 2
|
| New Jersey | NJ | Trenton | Newark | Dec 18, 1787 | 9,288,994 |
8,723 | 22,591 | 12
|
| New Mexico | NM | Santa Fe | Albuquerque | Jan 6, 1912 | 2,117,522 |
121,590 | 314,917 | 3
|
| New York | NY | Albany | New York City | Jul 26, 1788 | 20,201,249 |
54,555 | 141,297 | 26
|
| North Carolina | NC | Raleigh | Charlotte | Nov 21, 1789 | 10,439,388 |
53,819 | 139,391 | 14
|
| North Dakota | ND | Bismarck | Fargo | Nov 2, 1889 | 779,094 |
70,698 | 183,108 | 1
|
| Ohio | OH | Columbus | Mar 1, 1803 | 11,799,448 |
44,826 | 116,098 | 15
| |
| Oklahoma | OK | Oklahoma City | Nov 16, 1907 | 3,959,353 |
69,899 | 181,037 | 5
| |
| Oregon | OR | Salem | Portland | Feb 14, 1859 | 4,237,256 |
98,379 | 254,799 | 6
|
| Pennsylvania | PA | Harrisburg | Philadelphia | Dec 12, 1787 | 13,002,700 |
46,054 | 119,280 | 17
|
| Rhode Island | RI | Providence | May 29, 1790 | 1,097,379 |
1,545 | 4,001 | 2
| |
| South Carolina | SC | Columbia | Charleston | May 23, 1788 | 5,118,425 |
32,020 | 82,933 | 7
|
| South Dakota | SD | Pierre | Sioux Falls | Nov 2, 1889 | 886,667 |
77,116 | 199,729 | 1
|
| Tennessee | TN | Nashville | Jun 1, 1796 | 6,910,840 |
42,144 | 109,153 | 9
| |
| Texas | TX | Austin | Houston | Dec 29, 1845 | 29,145,505 |
268,596 | 695,662 | 38
|
| Utah | UT | Salt Lake City | Jan 4, 1896 | 3,271,616 |
84,897 | 219,882 | 4
| |
| Vermont | VT | Montpelier | Burlington | Mar 4, 1791 | 643,077 |
9,616 | 24,906 | 1
|
| Virginia | VA | Richmond | Virginia Beach | Jun 25, 1788 | 8,631,393 |
42,775 | 110,787 | 11
|
| Washington | WA | Olympia | Seattle | Nov 11, 1889 | 7,705,281 |
71,298 | 184,661 | 10
|
| West Virginia | WV | Charleston | Jun 20, 1863 | 1,793,716 |
24,230 | 62,756 | 2
| |
| Wisconsin | WI | Madison | Milwaukee | May 29, 1848 | 5,893,718 |
65,496 | 169,635 | 8
|
| Wyoming | WY | Cheyenne | Jul 10, 1890 | 576,851 |
97,813 | 253,335 | 1
|
## Federal district
| Flag, name and postal abbreviation |
Established | Population (2020) |
Total area | Reps. | ||
|---|---|---|---|---|---|---|
mi2 |
km2
| |||||
| District of Columbia | DC | Jul 16, 1790 | 689,545 | 68 | 176 | 1 |
## Territories
### Inhabited territories
| Name and postal abbreviation |
Capital | Acquired |
Territorial status | Population (2020) |
Total area | Reps. | ||
|---|---|---|---|---|---|---|---|---|
mi2 |
km2
| |||||||
| American Samoa | AS | Pago Pago | 1900 | 49,710 |
581 | 1,505 | 1
| |
| Guam | GU | Hagåtña | 1899 | Unincorporated, organized |
153,836 |
571 | 1,478 | 1
|
| Northern Mariana Islands | MP | Saipan | 1986 | Unincorporated, organized
|
47,329 |
1,976 | 5,117 | 1
|
| Puerto Rico | PR | San Juan | 1899 | Unincorporated, organized |
3,285,874 |
5,325 | 13,791 | 1
|
| U.S. Virgin Islands | VI | Charlotte Amalie | 1917 | Unincorporated, organized |
87,146 |
733 | 1,898 | 1
|
### Uninhabited territories
| Name | Acquired | Territorial status | Land area | |
|---|---|---|---|---|
mi2 |
km2
| |||
| Baker Island | 1856 | 0.9 | 2.2 | |
| Howland Island | 1858 | Unincorporated, unorganized |
0.6 | 1.6 |
| Jarvis Island | 1856 | Unincorporated, unorganized |
2.2 | 5.7 |
| Johnston Atoll | 1859 | Unincorporated, unorganized |
1 | 2.6 |
| Kingman Reef | 1860 | Unincorporated, unorganized |
0.005 | 0.01 |
| Midway Atoll | 1867 | Unincorporated, unorganized |
3 | 7.8 |
| Navassa Island | 1858 | Unincorporated, unorganized |
3 | 7.8 |
| Palmyra Atoll | 1898 | Incorporated, unorganized |
1.5 | 3.9 |
| Wake Island | 1899 | Unincorporated, unorganized |
2.5 | 6.5 |
### Disputed territories
| Name | Claimed |
Territorial status | Area | Administered by | Also claimed by | |
|---|---|---|---|---|---|---|
mi2 |
km2
| |||||
| Bajo Nuevo Bank (Petrel Island) | 1869 | Unincorporated, unorganized
(disputed sovereignty) |
56 | 145 | Colombia | Jamaica Nicaragua |
| Serranilla Bank | 1880 | Unincorporated, unorganized
(disputed sovereignty) |
463 | 1,200 | Colombia | Honduras Nicaragua |
## See also
- Aboriginal title in the United States
- Historic regions of the United States
- List of Indian reservations in the United States
- List of regions of the United States
- Lists of U.S. state topics
- List of U.S. states and territories by population
- Local government in the United States
- Organized incorporated territories of the United States
- Proposals for a 51st state
- Territorial evolution of the United States
- U.S. territorial sovereignty
- Compact of Free Association
## Explanatory notes
- Radan, Peter (2007).
*Creating New States: Theory and Practice of Secession*. Ashgate Publishing, Ltd. ISBN 9780754671633.
|
# Arthropod - Wikipedia
| Arthropods | |
|---|---|
| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
Clade:
|
ParaHoxozoa |
Clade:
|
Bilateria |
Clade:
|
Nephrozoa |
Clade:
|
Protostomia |
| Superphylum: | Ecdysozoa |
Clade:
|
Panarthropoda |
| Phylum: | Arthropoda Gravenhorst, 1843 |
| Living clades | |
|
For fossil groups, see text | |
| Diversity | |
| around 1,170,000 species | |
| Synonyms | |
|
**Arthropods** (*AR-thrə-pod*) are invertebrates in the phylum **Arthropoda**. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated (metameric) segments, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They form an extremely diverse group of up to ten million species.
Haemolymph is the analogue of blood for most arthropods. An arthropod has an open circulatory system, with a body cavity called a haemocoel through which haemolymph circulates to the interior organs. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. They have ladder-like nervous systems, with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.
Arthropods use combinations of compound eyes and pigment-pit ocelli for vision. In most species, the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many bristles known as setae that project through their cuticles. Similarly, their reproduction and development are varied; all terrestrial species use internal fertilization, but this is sometimes by indirect transfer of the sperm via an appendage or the ground, rather than by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, with many species giving birth to live young after the eggs have hatched inside the mother; but a few are genuinely viviparous, such as aphids. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by social insects.
The evolutionary ancestry of arthropods dates back to the Cambrian period. The group is generally regarded as monophyletic, and many analyses support the placement of arthropods with cycloneuralians (or their constituent clades) in a superphylum Ecdysozoa. Overall, however, the basal relationships of animals are not yet well resolved. Likewise, the relationships between various arthropod groups are still actively debated. Today, arthropods contribute to the human food supply both directly as food, and more importantly, indirectly as pollinators of crops. Some species are known to spread severe disease to humans, livestock, and crops.
The word *arthropod* comes from the Greek ἄρθρον *árthron* 'joint', and πούς *pous* (gen. ποδός *podos*) 'foot' or 'leg', which together mean "jointed leg", with the word "arthropodes" initially used in anatomical descriptions by Barthélemy Charles Joseph Dumortier published in 1832. The designation "Arthropoda" appears to have been first used in 1843 by the German zoologist Johann Ludwig Christian Gravenhorst (1777–1857). The origin of the name has been the subject of considerable confusion, with credit often given erroneously to Pierre André Latreille or Karl Theodor Ernst von Siebold instead, among various others.
Terrestrial arthropods are often called bugs. The term is also occasionally extended to colloquial names for freshwater or marine crustaceans (e.g., Balmain bug, Moreton Bay bug, mudbug) and used by physicians and bacteriologists for disease-causing germs (e.g., superbugs), but entomologists reserve this term for a narrow category of "true bugs", insects of the order Hemiptera.
Arthropods are invertebrates with segmented bodies and jointed limbs. The exoskeleton or cuticle consists of chitin, a polymer of N-Acetylglucosamine. The cuticle of many crustaceans, beetle mites, the clades Penetini and Archaeoglenini inside the beetle subfamily Phrenapatinae, and millipedes (except for bristly millipedes) is also biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments, also occurs in some opiliones, and the pupal cuticle of the fly *Bactrocera dorsalis* contains calcium phosphate.
Arthropoda is the largest animal phylum, with the estimates of the number of arthropod species varying from 1,170,000 to 5~10 million and accounting for over 80 percent of all known living animal species. One arthropod sub-group, the insects, includes more described species than any other taxonomic class. The total number of species remains difficult to determine, as estimates rely on census counts at specific locations, scaled up and projected onto other regions, then totalled - allowing for double-counting - to cover the whole world. Modeling assumptions are involved at each stage, introducing uncertainty. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater, land and air ecosystems and one of only two major animal groups that have adapted to life in dry environments; the other is amniotes, whose living members are reptiles, birds and mammals. Both the smallest and largest arthropods are crustaceans. The smallest belong to the class Tantulocarida, some of which are less than 100 micrometres (0.0039 in) long. The largest are species in the class Malacostraca, with the legs of the Japanese spider crab potentially spanning up to 4 metres (13 ft) and the American lobster reaching weights over 20 kg (44 lbs).
The embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways.
The three-part appearance of many insect bodies and the two-part appearance of spiders is a result of this grouping. There are no external signs of segmentation in mites. Arthropods also have two body elements that are not part of this serially repeated pattern of segments, an ocular somite at the front, where the mouth and eyes originated, and a telson at the rear, behind the anus.
Originally, it seems that each appendage-bearing segment had two separate pairs of appendages: an upper, unsegmented exite and a lower, segmented endopod. These would later fuse into a single pair of biramous appendages united by a basal segment (protopod or basipod), with the upper branch acting as a gill while the lower branch was used for locomotion. The appendages of most crustaceans and some extinct taxa such as trilobites have another segmented branch known as exopods, but whether these structures have a single origin remain controversial. In some segments of all known arthropods, the appendages have been modified, for example to form gills, mouth-parts, antennae for collecting information, or claws for grasping; arthropods are "like Swiss Army knives, each equipped with a unique set of specialized tools." In many arthropods, appendages have vanished from some regions of the body; it is particularly common for abdominal appendages to have disappeared or be highly modified.
The most conspicuous specialization of segments is in the head. The four major groups of arthropods – Chelicerata (sea spiders, horseshoe crabs and arachnids), Myriapoda (symphylans, pauropods, millipedes and centipedes), Pancrustacea (oligostracans, copepods, malacostracans, branchiopods, hexapods, etc.), and the extinct Trilobita – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways. Despite myriapods and hexapods both having similar head combinations, hexapods are deeply nested within crustacea while myriapods are not, so these traits are believed to have evolved separately. In addition, some extinct arthropods, such as *Marrella*, belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.
Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "The arthropod head problem". In 1960, R. E. Snodgrass even hoped it would not be solved, as he found trying to work out solutions to be fun.
Arthropod exoskeletons are made of cuticle, a non-cellular material secreted by the epidermis. Their cuticles vary in the details of their structure, but generally consist of three main layers: the epicuticle, a thin outer waxy coat that moisture-proofs the other layers and gives them some protection; the exocuticle, which consists of chitin and chemically hardened proteins; and the endocuticle, which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the procuticle. Each body segment and limb section is encased in hardened cuticle. The joints between body segments and between limb sections are covered by flexible cuticle.
The exoskeletons of most aquatic crustaceans are biomineralized with calcium carbonate extracted from the water. Some terrestrial crustaceans have developed means of storing the mineral, since on land they cannot rely on a steady supply of dissolved calcium carbonate. Biomineralization generally affects the exocuticle and the outer part of the endocuticle. Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor, and that it allows animals to grow larger and stronger by providing more rigid skeletons; and in either case a mineral-organic composite exoskeleton is cheaper to build than an all-organic one of comparable strength.
The cuticle may have setae (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages. For example, they are often used as sensors to detect air or water currents, or contact with objects; aquatic arthropods use feather-like setae to increase the surface area of swimming appendages and to filter food particles out of water; aquatic insects, which are air-breathers, use thick felt-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.
Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors; for example, all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.
The exoskeleton cannot stretch and thus restricts growth. Arthropods, therefore, replace their exoskeletons by undergoing ecdysis (moulting), or shedding the old exoskeleton, the exuviae, after growing a new one that is not yet hardened. Moulting cycles run nearly continuously until an arthropod reaches full size. The developmental stages between each moult (ecdysis) until sexual maturity is reached is called an instar. Differences between instars can often be seen in altered body proportions, colors, patterns, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again.
In the initial phase of moulting, the animal stops feeding and its epidermis releases moulting fluid, a mixture of enzymes that digests the endocuticle and thus detaches the old cuticle. This phase begins when the epidermis has secreted a new epicuticle to protect it from the enzymes, and the epidermis secretes the new exocuticle while the old cuticle is detaching. When this stage is complete, the animal makes its body swell by taking in a large quantity of water or air, and this makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest. It commonly takes several minutes for the animal to struggle out of the old cuticle. At this point, the new one is wrinkled and so soft that the animal cannot support itself and finds it very difficult to move, and the new endocuticle has not yet formed. The animal continues to pump itself up to stretch the new cuticle as much as possible, then hardens the new exocuticle and eliminates the excess air or water. By the end of this phase, the new endocuticle has formed. Many arthropods then eat the discarded cuticle to reclaim its materials.
Because arthropods are unprotected and nearly immobilized until the new cuticle has hardened, they are in danger both of being trapped in the old cuticle and of being attacked by predators. Moulting may be responsible for 80 to 90% of all arthropod deaths.
Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory, and excretory systems have repeated components. Arthropods come from a lineage of animals that have a coelom, a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a hydrostatic skeleton, which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems. Its place is largely taken by a hemocoel, a cavity that runs most of the length of the body and through which blood flows.
Arthropods have open circulatory systems. Most have a few short, open-ended arteries. In chelicerates and crustaceans, the blood carries oxygen to the tissues, while hexapods use a separate system of tracheae. Many crustaceans and a few chelicerates and tracheates use respiratory pigments to assist oxygen transport. The most common respiratory pigment in arthropods is copper-based hemocyanin; this is used by many crustaceans and a few centipedes. A few crustaceans and insects use iron-based hemoglobin, the respiratory pigment used by vertebrates. As with other invertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in corpuscles as they are in vertebrates.
The heart is a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Sections not being squeezed by the heart muscle are expanded either by elastic ligaments or by small muscles, in either case connecting the heart to the body wall. Along the heart run a series of paired ostia, non-return valves that allow blood to enter the heart but prevent it from leaving before it reaches the front.
Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have book lungs. Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and arachnids.
Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair of ganglia from which sensory and motor nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by commissures (relatively large bundles of nerves), which give arthropod nervous systems a characteristic ladder-like appearance. The brain is in the head, encircling and mainly above the esophagus. It consists of the fused ganglia of the acron and one or two of the foremost segments that form the head – a total of three pairs of ganglia in most arthropods, but only two in chelicerates, which do not have antennae or the ganglion connected to them. The ganglia of other head segments are often close to the brain and function as part of it. In insects, these other head ganglia combine into a pair of subesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as all the segmental ganglia are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment").
There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that metabolise nitrogen is ammonia, which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. All crustaceans use this system, and its high consumption of water may be responsible for the relative lack of success of crustaceans as land animals. Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is uric acid, which can be excreted as dry material; the Malpighian tubule system filters the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dumps these materials into the hindgut, from which they are expelled as feces. Most aquatic arthropods and some terrestrial ones also have organs called nephridia ("little kidneys"), which extract other wastes for excretion as urine.
The stiff cuticles of arthropods would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly setae, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of taste and smell, often by means of setae. Pressure sensors often take the form of membranes that function as eardrums, but are connected directly to nerves rather than to auditory ossicles. The antennae of most hexapods include sensor packages that monitor humidity, moisture and temperature.
Most arthropods lack balance and acceleration sensors, and rely on their eyes to tell them which way is up. The self-righting behavior of cockroaches is triggered when pressure sensors on the underside of the feet report no pressure. However, many malacostracan crustaceans have statocysts, which provide the same sort of information as the balance and motion sensors of the vertebrate inner ear.
The proprioceptors of arthropods, sensors that report the force exerted by muscles and the degree of bending in the body and joints, are well understood. However, little is known about what other internal sensors arthropods may have.
Most arthropods have sophisticated visual systems that include one or more usually both of compound eyes and pigment-cup ocelli ("little eyes"). In most cases, ocelli are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However, the main eyes of spiders are pigment-cup ocelli that are capable of forming images, and those of jumping spiders can rotate to track prey.
Compound eyes consist of fifteen to several thousand independent ommatidia, columns that are usually hexagonal in cross section. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens and cornea. Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the polarization of light. On the other hand, the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals – although this is not a severe disadvantage, as objects and events within 20 cm (8 in) are most important to most arthropods. Several arthropods have color vision, and that of some insects has been studied in detail; for example, the ommatidia of bees contain receptors for both green and ultra-violet.
A few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex their entire lives. A few species of insects and crustaceans can reproduce by parthenogenesis, especially if conditions favor a "population explosion". However, most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable. The ability to undergo meiosis is widespread among arthropods including both those that reproduce sexually and those that reproduce parthenogenetically. Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem, that appears to have remained unsettled.
Aquatic arthropods may breed by external fertilization, as for example horseshoe crabs do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization. Opiliones (harvestmen), millipedes, and some crustaceans use modified appendages such as gonopods or penises to transfer the sperm directly to the female. However, most male terrestrial arthropods produce spermatophores, waterproof packets of sperm, which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely to be successful.
Most arthropods lay eggs, but scorpions are ovoviviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care. Newly born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first moult. Many insects hatch as grubs or caterpillars, which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body. Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws. Crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages.
Based on the distribution of shared plesiomorphic features in extant and fossil taxa, the last common ancestor of all arthropods is inferred to have been as a modular organism with each module covered by its own sclerite (armor plate) and bearing a pair of biramous limbs. However, whether the ancestral limb was uniramous or biramous is far from a settled debate. This Ur-arthropod had a ventral mouth, pre-oral antennae and dorsal eyes at the front of the body. It was assumed to have been a non-discriminatory sediment feeder, processing whatever sediment came its way for food, but fossil findings hint that the last common ancestor of both arthropods and Priapulida shared the same specialized mouth apparatus: a circular mouth with rings of teeth used for capturing animal prey.
It has been proposed that the Ediacaran animals *Parvancorina* and *Spriggina*, from around , were arthropods, but later study shows that their affinities of being origin of arthropods are not reliable. Small arthropods with bivalve-like shells have been found in Early Cambrian fossil beds dating in China and Australia. The earliest Cambrian trilobite fossils are about 520 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time. In the Maotianshan shales, which date back to 518 million years ago, arthropods such as *Kylinxia* and *Erratus* have been found that seem to represent transitional fossils between stem (e.g. Radiodonta such as *Anomalocaris*) and true arthropods. Re-examination in the 1970s of the Burgess Shale fossils from about identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about the Cambrian explosion. A fossil of *Marrella* from the Burgess Shale has provided the earliest clear evidence of moulting.
The earliest fossil of likely pancrustacean larvae date from about in the Cambrian, followed by unique taxa like *Yicaris* and *Wujicaris*. The purported pancrustacean/crustacean affinity of some cambrian arthropods (e.g. Phosphatocopina, Bradoriida and Hymenocarine taxa like waptiids) were disputed by subsequent studies, as they might branch before the mandibulate crown-group. Within the pancrustacean crown-group, only Malacostraca, Branchiopoda and Pentastomida have Cambrian fossil records. Crustacean fossils are common from the Ordovician period onwards. They have remained almost entirely aquatic, possibly because they never developed excretory systems that conserve water.
Arthropods provide the earliest identifiable fossils of land animals, from about Silurian, and terrestrial tracks from about appear to have been made by arthropods. Arthropods possessed attributes that were easy coopted for life on land; their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water. Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 m (8 ft 2 in).
in the LateThe oldest known arachnid is the trigonotarbid *Palaeotarbus jerami*, from about in the Silurian period. *Attercopus fimbriunguis*, from in the Devonian period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true spiders, which first appear in the Late Carboniferous over . The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families. The oldest known scorpion is *Dolichophonus,* dated back to . Lots of Silurian and Devonian scorpions were previously thought to be gill-breathing, hence the idea that scorpions were primitively aquatic and evolved air-breathing book lungs later on. However subsequent studies reveal most of them lacking reliable evidence for an aquatic lifestyle, while exceptional aquatic taxa (e.g. *Waeringoscorpio*) most likely derived from terrestrial scorpion ancestors.
The oldest fossil record of hexapod is obscure, as most of the candidates are poorly preserved and their hexapod affinities had been disputed. An iconic example is the Devonian *Rhyniognatha hirsti*, dated at , its mandibles are thought to be a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period. However later study shows that *Rhyniognatha* most likely represent a myriapod, not even a hexapod. The unequivocal oldest known hexapod is the springtail *Rhyniella*, from about in the Devonian period, and the palaeodictyopteran *Delitzschala bitterfeldensis*, from about in the Carboniferous period, respectively. The Mazon Creek lagerstätten from the Late Carboniferous, about , include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle Cenozoic.
From 1952 to 1977, zoologist Sidnie Manton and others argued that arthropods are polyphyletic, in other words, that they do not share a common ancestor that was itself an arthropod. Instead, they proposed that three separate groups of "arthropods" evolved separately from common worm-like ancestors: the chelicerates, including spiders and scorpions; the crustaceans; and the uniramia, consisting of onychophorans, myriapods and hexapods. These arguments usually bypassed trilobites, as the evolutionary relationships of this class were unclear. Proponents of polyphyly argued the following: that the similarities between these groups are the results of convergent evolution, as natural consequences of having rigid, segmented exoskeletons; that the three groups use different chemical means of hardening the cuticle; that there were significant differences in the construction of their compound eyes; that it is hard to see how such different configurations of segments and appendages in the head could have evolved from the same ancestor; and that crustaceans have biramous limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg.
Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are monophyletic, in other words they are inferred to share a common ancestor that was itself an arthropod. For example, Graham Budd's analyses of *Kerygmachela* in 1993 and of *Opabinia* in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods. These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "Panarthropoda" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods").
A contrary view was presented in 2003, when Jan Bergström and Hou Xian-guang argued that, if arthropods were a "sister-group" to any of the anomalocarids, they must have lost and then re-evolved features that were well-developed in the anomalocarids. The earliest known arthropods ate mud in order to extract food particles from it, and possessed variable numbers of segments with unspecialized appendages that functioned as both gills and legs. Anomalocarids were, by the standards of the time, huge and sophisticated predators with specialized mouths and grasping appendages, fixed numbers of segments some of which were specialized, tail fins, and gills that were very different from those of arthropods. In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids. In 2014, it was found that tardigrades were more closely related to arthropods than velvet worms.
Higher up the "family tree", the Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelled Articulata. There had been competing proposals that arthropods were closely related to other groups such as nematodes, priapulids and tardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups.
In the 1990s, molecular phylogenetic analyses of DNA sequences produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that moult"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present at all in arthropods. This hypothesis groups annelids with molluscs and brachiopods in another superphylum, Lophotrochozoa.
If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved convergently or has been inherited from a much older ancestor and subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.
| Arthropod fossil phylogeny |
| Summarized cladogram of the relationships between extinct arthropod groups. For more, see Deuteropoda. |
Aside from the four major living groups (crustaceans, chelicerates, myriapods and hexapods), a number of fossil forms, mostly from the early Cambrian period, are difficult to place taxonomically, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them. *Marrella* was the first one to be recognized as significantly different from the well-known groups.
Modern interpretations of the basal, extinct stem-group of Arthropoda recognised the following groups, from most basal to most crownward:
- The "Giant" or "Siberiid Lobopodians", such as
*Jianshanopodia*,*Siberion*and*Megadictyon*, are the most basal grade in the total-group Arthropoda. - The "Gilled Lobopodians", such as
*Kerygmachela*,*Pambdelurion*and*Opabinia*, are the second most basal grade. - The Radiodonta, which traditionally known as anomalocaridids come in third position, and are thought to be monophyletic.
- A possible "upper stem-group" assemblage of more uncertain position but contained within Deuteropoda: the Fuxianhuiida, Megacheira, and multiple "bivalved forms" including Isoxyida and Hymenocarina.
The Deuteropoda is a recently established clade uniting the crown-group (living) arthropods with these possible "upper stem-group" fossils taxa. The clade is defined by important changes to the structure of the head region such as the appearance of a differentiated deutocerebral appendage pair, which excludes more basal taxa like radiodonts and "gilled lobopodians".
Controversies remain about the positions of various extinct arthropod groups. Some studies recover Megacheira as closely related to chelicerates, while others recover them as outside the group containing Chelicerate and Mandibulata as stem-group euarthropods. The placement of the Artiopoda (which contains the extinct trilobites and similar forms) is also a frequent subject of dispute. The main hypotheses position them in the clade Arachnomorpha with the Chelicerates. However, one of the newer hypotheses is that the chelicerae have originated from the same pair of appendages that evolved into antennae in the ancestors of Mandibulata, which would place trilobites, which had antennae, closer to Mandibulata than Chelicerata, in the clade Antennulata. The fuxianhuiids, usually suggested to be stem-group arthropods, have been suggested to be Mandibulates in some recent studies. The Hymenocarina, a group of bivalved arthropods, previously thought to have been stem-group members of the group, have been demonstrated to be mandibulates based on the presence of mandibles.
List of arthropod groups and genera († denotes extinct taxa)
- "Dinocaridida" † (generally considered paraphyletic, sometimes treated as lobopodians)
- Kerygmachelidae †
*Pambdelurion*† (possible lobopodian)*Mieridduryn*† (possible opabiniid)*Parvibellus*† (possible "Siberiid Lobopodian")- Opabiniidae †
- Radiodonta †
*Cucumericrus*† (possible radiodont)*Caryosyntrips*† (possible radiodont)
- Deuteropoda
- Artiopoda †
- Trilobita †
- Agnostida (possibly trilobites) †
- Nektaspida †
- Aglaspidida †
- Cheloniellida †
*Bushizheia*†*Erratus*†*Fengzhengia*†- Fuxianhuiida †
- Isoxyida †
*Kiisortoqia*†*Kylinxia*†- Marrellomorpha †
- Bradoriida †
- Megacheira † (possibly paraphyletic, alternatively placed as stem-chelicerates)
- Chelicerata
- Habeliida †
- Pycnogonida
- Prosomapoda
- "Synziphosurina" (paraphyletic)
- Xiphosura
- Dekatriata
- Phosphatocopina (possible stem mandibulate) †
- Mandibulata
- Artiopoda †
*Incertae sedis**Aaveqaspis*†*Arthrogyrinus*†*Bennettarthra*†*Burgessia*†- Cambropachycopidae †
*Cambropodus*†*Camptophyllia*†*Chuandianella*†*Keurbos*†*Notchia*†*Parioscorpio*†*Pleuralata*†*Rhynimonstrum*†*Sarotrocercus*†- Strabopida †
- Sunellidae †
*Wingertshellicus*†*Zhenghecaris*†
The phylum Arthropoda is typically subdivided into four subphyla, of which one is extinct:
**Artiopods**are an extinct group of formerly numerous marine arthropods that disappeared in the Permian–Triassic extinction event, though they were in decline prior to this killing blow, having been reduced to a handful of orders in the Late Devonian extinction. They contain groups such as the trilobites, nektaspids, aglaspidids, and the cheloniellids among others.**Chelicerates**comprise the marine sea spiders and horseshoe crabs, along with the terrestrial arachnids such as mites, harvestmen, spiders, scorpions and related organisms characterized by the presence of chelicerae, appendages just above/in front of the mouthparts. Chelicerae appear in scorpions and horseshoe crabs as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom.**Myriapods**comprise millipedes, centipedes, pauropods and symphylans, characterized by having numerous body segments each of which bearing one or two pairs of legs (or in a few cases being legless). All members are exclusively terrestrial.**Pancrustaceans**comprise ostracods, barnacles, copepods, malacostracans, cephalocaridans, branchiopods, remipedes and hexapods. Most groups are primarily aquatic (two notable exceptions being woodlice and hexapods, which are both purely terrestrial) and are characterized by having biramous appendages. The most abundant group of pancrustaceans are the terrestrial hexapods, which comprise insects, diplurans, springtails, and proturans, with six thoracic legs.
The phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute. Recent studies strongly suggest that Crustacea, as traditionally defined, is paraphyletic, with Hexapoda having evolved from within it, so that Crustacea and Hexapoda form a clade, Pancrustacea. The position of Myriapoda, Chelicerata and Pancrustacea remains unclear as of April 2012[update]. In some studies, Myriapoda is grouped with Chelicerata (forming Myriochelata); in other studies, Myriapoda is grouped with Pancrustacea (forming Mandibulata), or Myriapoda may be sister to Chelicerata plus Pancrustacea.
The following cladogram shows the internal relationships between all the living classes of arthropods as of the late 2010s, as well as the estimated timing for some of the clades:
Arthropoda
|
||
Crustaceans such as crabs, lobsters, crayfish, shrimp, and prawns have long been part of human cuisine, and are now raised commercially. Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many cultures, though not most European, Hindu, and Islamic cultures. Cooked tarantulas are considered a delicacy in Cambodia, and by the Piaroa Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed. Humans also unintentionally eat arthropods in other foods, and food safety regulations lay down acceptable contamination levels for different kinds of food material. The intentional cultivation of arthropods and other small animals for human food, referred to as minilivestock, is now emerging in animal husbandry as an ecologically sound concept. Commercial butterfly breeding provides Lepidoptera stock to butterfly conservatories, educational exhibits, schools, research facilities, and cultural events.
However, the greatest contribution of arthropods to human food supply is by pollination: a 2008 study examined the 100 crops that FAO lists as grown for food, and estimated pollination's economic value as €153 billion, or 9.5 per cent of the value of world agricultural production used for human food in 2005. Besides pollinating, bees produce honey, which is the basis of a rapidly growing industry and international trade.
The red dye cochineal, produced from a Central American species of insect, was economically important to the Aztecs and Mayans. While the region was under Spanish control, it became Mexico's second most-lucrative export, and is now regaining some of the ground it lost to synthetic competitors. Shellac, a resin secreted by a species of insect native to southern Asia, was historically used in great quantities for many applications in which it has mostly been replaced by synthetic resins, but it is still used in woodworking and as a food additive. The blood of horseshoe crabs contains a clotting agent, Limulus Amebocyte Lysate, which is now used to test that antibiotics and kidney machines are free of dangerous bacteria, and to detect spinal meningitis. Forensic entomology uses evidence provided by arthropods to establish the time and sometimes the place of death of a human, and in some cases the cause. Recently insects have also gained attention as potential sources of drugs and other medicinal substances.
The relative simplicity of the arthropods' body plan, allowing them to move on a variety of surfaces both on land and in water, have made them useful as models for robotics. The redundancy provided by segments allows arthropods and biomimetic robots to move normally even with damaged or lost appendages.
| Disease | Insect | Cases per year | Deaths per year |
|---|---|---|---|
| Malaria | Anopheles mosquito |
267 M | 1 to 2 M |
| Dengue fever | Aedes mosquito |
5 M | 5,000 |
| Yellow fever | Aedes mosquito |
4,432 | 1,177 |
| Filariasis | Culex mosquito |
250 M | unknown |
Although arthropods are the most numerous phylum on Earth, and thousands of arthropod species are venomous, they inflict relatively few serious bites and stings on humans. Far more serious are the effects on humans of diseases like malaria carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others. Ticks can cause tick paralysis and several parasite-borne diseases in humans. A few of the closely related mites also infest humans, causing intense itching, and others cause allergic diseases, including hay fever, asthma, and eczema.
Many species of arthropods, principally insects but also mites, are agricultural and forest pests. The mite *Varroa destructor* has become the largest single problem faced by beekeepers worldwide. Efforts to control arthropod pests by large-scale use of pesticides have caused long-term effects on human health and on biodiversity. Increasing arthropod resistance to pesticides has led to the development of integrated pest management using a wide range of measures including biological control. Predatory mites may be useful in controlling some mite pests.
- Gould, S. J. (1990).
*Wonderful Life: The Burgess Shale and the Nature of History*. Hutchinson Radius. Bibcode:1989wlbs.book.....G. ISBN 978-0-09-174271-3. - Ruppert, E. E.; R. S. Fox; R. D. Barnes (2004).
*Invertebrate Zoology*(7th ed.). Brooks/Cole. ISBN 978-0-03-025982-1.
- "
*Arthropod*".*The Encyclopedia of Life*. - Venomous Arthropods Archived 31 January 2010 at the Wayback Machine chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
- Arthropods – Arthropoda Insect Life Forms
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