A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma
The Plains bison is one of two subspecies/ecotypes of the American bison, the other being the wood bison. A natural population of Plains bison survives in Yellowstone National Park and multiple smaller reintroduced herds of bison in many places in Canada and the United States. At one time, at least 25 million American bison were spread across the United States and Canada. However, by the late 1880s, the total number of bison in the United States had been reduced to fewer than 600 individuals. Most of these were collected onto various private ranches, the last known free-roaming population of bison consisted of less than 30 in the area which became Yellowstone National Park. Though it was the official policy of the United States government to minimize or eliminate the bison, most farmers and ranchers considered bison to be a pest or nuisance, some people were concerned about the demise of this North American icon, so individual landowners took steps to protect a few; some people saved bison with the express purpose of ranching or hunting them, but some groups such as the American Bison Society were formed with the idea of saving the species and reintroducing them to at least part of their previous natural range.
Plains bison have since been reintroduced into a number of locations around North America. Five main foundation herds of American bison supplied; the northernmost introduction occurred in 1928 when the Alaska Game Commission brought bison to the area of present-day Delta Junction. Bison taken from this transplant were introduced to other Alaska locations, including Farewell and Chitina; the Delta Junction herd prospered the most, with a population of several hundred throughout the late 20th century. This herd is popular with hunters interested in hundreds of pounds of high-quality meat, but has been a problem for farming operations in the area. Though American bison prefer grasslands and plains habitats, they are quite adaptable and live in conditions ranging from desert, as in the case of the Henry Mountains bison herd, to forested areas, such as those of the Yellowstone Park bison herd. Over 500,000 bison are spread over the United States and Canada. However, most of these are on private ranches, some of them have small amounts of hybridized cattle genes.
Significant public bison herds that do not appear to have hybridized domestic cattle genes are the Yellowstone Park, the Henry Mountains, the Wind Cave, the Wood Buffalo National Park bison herds and subsidiary herds descended from it in Canada. Park officials transferred Plains bison from Fort Niobrara National Wildlife Refuge to Theodore Roosevelt National Park's South Unit in 1956 and its North Unit in 1962 for population increase. In 1969, Plains bison from Elk Island National Park were released into Prince Albert National Park in Saskatchewan, creating the Sturgeon River bison herd. At a population around 300 animals, they form a free herd able to wander; the bison are spread throughout Prince Albert National Park's southwestern corner, as well as some crown and private land in the area. In 2006, Plains bison from Elk Island National Park in Alberta were released into Saskatchewan's Grasslands National Park; this marks the first time Plains bison have wandered the shortgrass prairies of Canada since their near-extinction at the turn of the 20th century.
According to the national agency Parks Canada, the entire breeding population of these wild and "semiwild" bison are the descendants of just eight individuals that survived the period of near-extinction, due to overhunting and tuberculosis infecting the herd that the government belatedly attempted to conserve. A herd of about 650 of these animals lives in, can be seen at, the Wichita Mountains Wildlife Refuge near Lawton, Oklahoma; the herd was started in 1907 with stock from the New York Zoological Park, now known as the Bronx Zoo and located in the Bronx Park. Fifteen animals were shipped to Oklahoma, where bison had become extinct due to excessive hunting and overharvesting by non-native commercial buffalo hunters from 1874 to 1878; some of these specimens have been released in other areas of the United States, such as Paynes Prairie in Florida. Only one Southern Plains bison herd was established in Texas. A remnant of the last of this relict herd had been saved in 1876. "Molly" Goodnight had encouraged her rancher husband, Charles Goodnight, to save some of the last bison which were taking refuge in the Texas Panhandle.
By saving these few Plains bison, she was able to establish a buffalo herd near the Palo Duro Canyon. This herd peaked at 250 in 1933. Bison of this herd were introduced into the Yellowstone National Park in 1902 and into the larger zoos and ranches throughout the nation. A herd of around 80 of these animals lives in the Caprock Canyons State Park near Quitaque, located about 50 miles northeast of Plainview, Texas. In 2013, bison were reintroduced to Fort Belknap Indian Reservation from Yellowstone National Park. A herd of Plains bison were reintroduced to Banff National Park in Alberta in early 2017; the bison are to be kept under observation in an enclosed pasture of the park until the summer of 2018, according to Parks Canada. Besides using the meat and organs for food, Plains tribes have traditionally created a wide variety of tools and items from bison; these include arrow points, beads, berry pounders, hide scrapers, needles from bones, spoons from the horns, bow strings and thread from the sinew, waterproof containers from the bladder, paint brushes from the tail and bones with intact marro
The United States of America known as the United States or America, is a country composed of 50 states, a federal district, five major self-governing territories, various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is smaller than the entire continent of Europe's 3.9 million square miles. With a population of over 327 million people, the U. S. is the third most populous country. The capital is Washington, D. C. and the largest city by population is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico; the State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean; the U. S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The diverse geography and wildlife of the United States make it one of the world's 17 megadiverse countries.
Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago. European colonization began in the 16th century; the United States emerged from the thirteen British colonies established along the East Coast. Numerous disputes between Great Britain and the colonies following the French and Indian War led to the American Revolution, which began in 1775, the subsequent Declaration of Independence in 1776; the war ended in 1783 with the United States becoming the first country to gain independence from a European power. The current constitution was adopted in 1788, with the first ten amendments, collectively named the Bill of Rights, being ratified in 1791 to guarantee many fundamental civil liberties; the United States embarked on a vigorous expansion across North America throughout the 19th century, acquiring new territories, displacing Native American tribes, admitting new states until it spanned the continent by 1848. During the second half of the 19th century, the Civil War led to the abolition of slavery.
By the end of the century, the United States had extended into the Pacific Ocean, its economy, driven in large part by the Industrial Revolution, began to soar. The Spanish–American War and World War I confirmed the country's status as a global military power; the United States emerged from World War II as a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, a permanent member of the United Nations Security Council. Sweeping civil rights legislation, notably the Civil Rights Act of 1964, the Voting Rights Act of 1965 and the Fair Housing Act of 1968, outlawed discrimination based on race or color. During the Cold War, the United States and the Soviet Union competed in the Space Race, culminating with the 1969 U. S. Moon landing; the end of the Cold War and the collapse of the Soviet Union in 1991 left the United States as the world's sole superpower. The United States is the world's oldest surviving federation, it is a representative democracy.
The United States is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, other international organizations. The United States is a developed country, with the world's largest economy by nominal GDP and second-largest economy by PPP, accounting for a quarter of global GDP; the U. S. economy is post-industrial, characterized by the dominance of services and knowledge-based activities, although the manufacturing sector remains the second-largest in the world. The United States is the world's largest importer and the second largest exporter of goods, by value. Although its population is only 4.3% of the world total, the U. S. holds 31% of the total wealth in the world, the largest share of global wealth concentrated in a single country. Despite wide income and wealth disparities, the United States continues to rank high in measures of socioeconomic performance, including average wage, human development, per capita GDP, worker productivity.
The United States is the foremost military power in the world, making up a third of global military spending, is a leading political and scientific force internationally. In 1507, the German cartographer Martin Waldseemüller produced a world map on which he named the lands of the Western Hemisphere America in honor of the Italian explorer and cartographer Amerigo Vespucci; the first documentary evidence of the phrase "United States of America" is from a letter dated January 2, 1776, written by Stephen Moylan, Esq. to George Washington's aide-de-camp and Muster-Master General of the Continental Army, Lt. Col. Joseph Reed. Moylan expressed his wish to go "with full and ample powers from the United States of America to Spain" to seek assistance in the revolutionary war effort; the first known publication of the phrase "United States of America" was in an anonymous essay in The Virginia Gazette newspaper in Williamsburg, Virginia, on April 6, 1776. The second draft of the Articles of Confederation, prepared by John Dickinson and completed by June 17, 1776, at the latest, declared "The name of this Confederation shall be the'United States of America'".
The final version of the Articles sent to the states for ratification in late 1777 contains the sentence "The Stile of this Confederacy shall be'The United States of America'". In June 1776, Thomas Jefferson wrote the phrase "UNITED STATES OF AMERICA" in all capitalized letters in the headline of his "original Rough draught" of the Declaration of Independence; this draft of the document did not surface unti
Galliformes is an order of heavy-bodied ground-feeding birds that includes turkey, chicken, New World quail and Old World quail, partridge, francolin and the Cracidae. The name derives from "gallus", Latin for "cock" or "rooster". Common names are gamefowl or gamebirds, gallinaceous birds, or galliforms. "Wildfowl" or just "fowl" are often used for the Galliformes, but these terms refer to waterfowl, to other hunted birds. This group has about 290 species, one or more of which are found in every part of the world's continents, they are rarer on islands, in contrast to the related waterfowl, are absent from oceanic islands—unless introduced there by humans. Several species have been domesticated during their extensive relationships with humans; this order contains five families: Phasianidae, Numididae and Megapodiidae. They are important as seed dispersers and predators in the ecosystems they inhabit, are reared as game birds by humans for their meat and eggs and for recreational hunting. Many gallinaceous species are skilled runners and escape predators by running rather than flying.
Males of most species are more colorful than the females. Males have elaborate courtship behaviors that include strutting, fluffing of tail or head feathers, vocal sounds, they are nonmigratory. The living Galliformes were once divided into seven or more families. Despite their distinctive appearance and turkeys do not warrant separation as families due to their recent origin from partridge- or pheasant-like birds; the turkeys became larger after their ancestors colonized temperate and subtropical North America, where pheasant-sized competitors were absent. The ancestors of grouse, adapted to harsh climates and could thereby colonize subarctic regions; the Phasianidae are expanded in current taxonomy to include the former Tetraonidae and Meleagrididae as subfamilies. The Anseriformes and the Galliformes together make up the Galloanserae, they are basal among the living neognathous birds, follow the Paleognathae in modern bird classification systems. This was first proposed in the Sibley-Ahlquist taxonomy and has been the one major change of that proposed scheme, universally adopted.
However, the Galliformes as they were traditionally delimited are called Gallomorphae in the Sibley-Ahlquist taxonomy, which splits the Cracidae and Megapodiidae as an order "Craciformes". This is not a natural group, but rather an erroneous result of the now-obsolete phenetic methodology employed in the Sibley-Ahlquist taxonomy. Phenetic studies do not distinguish between plesiomorphic and apomorphic characters, which leads to basal lineages appearing as monophyletic groups; the buttonquails and the hoatzin were placed in the Galliformes, too. The former are now known to be shorebirds adapted to an inland lifestyle, whereas the mesites are closely related to pigeons and doves; the relationships of the hoatzin are obscure, it is treated as a monotypic order Opisthocomiformes to signify this. Galliform-like birds were one of the main survivors of the K-T Event that killed off the rest of the dinosaurs, they were a niche group that were toothless and ground-dwelling, unlike the dominant birds of the era called the enantiornithes, which had teeth and dominated the trees and skies.
Fossils of these galliform-like birds originate in the Late Cretaceous, most notably those of Austinornis lentus. Its partial left tarsometatarsus was found in the Austin Chalk near Fort McKinney, dating to about 85 million years ago; this bird was quite closely related to Galliformes, but whether it was a part of these or belongs elsewhere in the little-known galliform branch of Galloanserae is not clear. However, in 2004, Clarke classified it as a member of the larger group Pangalliformes, more related to chickens than to ducks, but not a member of the crown group that includes all modern galliformes. Another specimen, PVPH 237, from the Late Cretaceous Portezuelo Formation in the Sierra de Portezuelo has been suggested to be an early galliform relative; this is a partial coracoid of a neornithine bird, which in its general shape and the wide and deep attachment for the muscle joining the coracoid and the humerus bone resembles the more basal lineages of galliforms. It is believed that an asteroid impact killed off all dinosaurs, including the dominant birds, during the K-T event, destroying all creatures that lived in trees and on open ground.
While the more successful enantiornithes were wiped out, the ancestors of galliformes were small and lived in the ground or water. This protected them from the destruction. Additional galliform-like pangalliformes are represented by extinct families from the Paleogene, namely the Gallinuloididae and Quercymegapodiidae. In the early Cenozoic, some additional birds may or may not be early Galliformes, though if they are, they are unlikely to belong to extant families: †Argillipes †Coturnipes †Paleophasianus †Percolinus †Amitabha (Bridger middle Eocene
An endangered species is a species, categorized as likely to become extinct. Endangered, as categorized by the International Union for Conservation of Nature Red List, is the second most severe conservation status for wild populations in the IUCN's schema after Critically Endangered. In 2012, the IUCN Red List featured 3,079 animal and 2,655 plant species as endangered worldwide; the figures for 1998 were 1,102 and 1,197. Many nations have laws that protect conservation-reliant species: for example, forbidding hunting, restricting land development or creating preserves. Population numbers and species' conservation status can be found at the lists of organisms by population; the conservation status of a species indicates the likelihood. Many factors are considered; the IUCN Red List of Threatened Species is the best-known worldwide conservation status listing and ranking system. Over 50% of the world's species are estimated to be at risk of extinction. Internationally, 199 countries have signed an accord to create Biodiversity Action Plans that will protect endangered and other threatened species.
In the United States, such plans are called Species Recovery Plans. Though labelled a list, the IUCN Red List is a system of assessing the global conservation status of species that includes "Data Deficient" species – species for which more data and assessment is required before their status may be determined – as well species comprehensively assessed by the IUCN's species assessment process; those species of "Near Threatened" and "Least Concern" status have been assessed and found to have robust and healthy populations, though these may be in decline. Unlike their more general use elsewhere, the List uses the terms "endangered species" and "threatened species" with particular meanings: "Endangered" species lie between "Vulnerable" and "Critically Endangered" species, while "Threatened" species are those species determined to be Vulnerable, Endangered or Critically Endangered; the IUCN categories, with examples of animals classified by them, include: Extinct no remaining individuals of the species Extinct in the wild Captive individuals survive, but there is no free-living, natural population.
Critically endangered Faces an high risk of extinction in the immediate future. Endangered Faces a high risk of extinction in the near future. Vulnerable Faces a high risk of endangerment in the medium term. Near-threatened May be considered threatened in the near future. Least concern No immediate threat to species' survival. A) Reduction in population size based on any of the following: An observed, inferred or suspected population size reduction of ≥ 70% over the last 10 years or three generations, whichever is the longer, where the causes of the reduction are reversible AND understood AND ceased, based on any of the following: direct observation an index of abundance appropriate for the taxon a decline in area of occupancy, extent of occurrence or quality of habitat actual or potential levels of exploitation the effects of introduced taxa, pathogens, competitors or parasites. An observed, inferred or suspected population size reduction of ≥ 50% over the last 10 years or three generations, whichever is the longer, where the reduction or its causes may not have ceased OR may not be understood OR may not be reversible, based on any of to under A1.
A population size reduction of ≥ 50%, projected or suspected to be met within the next 10 years or three generations, whichever is the longer, based on any of to under A1. An observed, inferred, projected or suspected population size reduction of ≥ 50% over any 10 year or three generation period, whichever is longer, where the time period must include both the past and the future, where the reduction or its causes may not have ceased OR may not be understood OR may not be reversible, based on any of to under A1. B) Geographic range in the form of either B1 OR B2 OR both: Extent of occurrence estimated to be less than 5,000 km², estimates indicating at least two of a-c: Severely fragmented or known to exist at no more than five locations. Continuing decline, observed or projected, in any of the following: extent of occurrence area of occupancy area, extent or quality of habitat number of locations or subpopulations number of mature individuals Extreme fluctuations in any of the following: extent of occurrence area of occupancy number of locations or subpopulations number of mature individuals Area of occupancy estimated to be less than 500 km², estimates indicating at least two of a-c: Severely fragmented or known to exist at no more than five locations.
Continuing decline, observed or projected, in any of the following: extent of occurrence area of occupancy area, extent or quality of habitat number of locations or subpopulations number of mature individuals Extreme fluctuations in any of the following: extent of occurrence area of occupancy number of locations or subpopulations number of mature individualsC) Population estimated to number fewer than 2,500 mature individuals and either: An estimated continuing decline of at least 20% within five years or two generations, whichever is longer, OR A continuing decline, projected
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with 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 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 animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
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, 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 develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction 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. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Owls are birds from the order Strigiformes, which includes about 200 species of solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, feathers adapted for silent flight. Exceptions include the gregarious burrowing owl. Owls hunt small mammals and other birds, although a few species specialize in hunting fish, they are found in all regions of the Earth except some remote islands. Owls are divided into two families: the true owl family and the barn-owl family, Tytonidae. Owls possess large, forward-facing eyes and ear-holes, a hawk-like beak, a flat face, a conspicuous circle of feathers, a facial disc, around each eye; the feathers making up this disc can be adjusted to focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey have eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting.
Although owls have binocular vision, their large eyes are fixed in their sockets—as are those of most other birds—so they must turn their entire heads to change views. As owls are farsighted, they are unable to see anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes—hairlike feathers on the beak and feet that act as "feelers", their far vision in low light, is exceptionally good. Owls can rotate their heads and necks as much as 270°. Owls have 14 neck vertebrae compared to seven in humans, they have adaptations to their circulatory systems, permitting rotation without cutting off blood to the brain: the foramina in their vertebrae through which the vertebral arteries pass are about 10 times the diameter of the artery, instead of about the same size as the artery as in humans. Other anastomoses between the carotid and vertebral arteries support this effect; the smallest owl—weighing as little as 31 g and measuring some 13.5 cm —is the elf owl.
Around the same diminutive length, although heavier, are the lesser known long-whiskered owlet and Tamaulipas pygmy owl. The largest owls are two sized eagle owls; the largest females of these species are 71 cm long, have 54 cm long wings, weigh 4.2 kg. Different species of owls produce different sounds; as noted above, their facial discs help owls to funnel the sound of prey to their ears. In many species, these discs are placed asymmetrically, for better directional location. Owl plumage is cryptic, although several species have facial and head markings, including face masks, ear tufts, brightly coloured irises; these markings are more common in species inhabiting open habitats, are thought to be used in signaling with other owls in low-light conditions. Sexual dimorphism is a physical difference between females of a species. Reverse sexual dimorphism, when females are larger than males, has been observed across multiple owl species; the degree of size dimorphism varies across multiple populations and species, is measured through various traits, such as wing span and body mass.
Overall, female owls tend to be larger than males. The exact explanation for this development in owls is unknown. However, several theories explain the development of sexual dimorphism in owls. One theory suggests that selection has led males to be smaller because it allows them to be efficient foragers; the ability to obtain more food is advantageous during breeding season. In some species, female owls stay at their nest with their eggs while it is the responsibility of the male to bring back food to the nest. However, if food is scarce, the male first feeds himself before feeding the female. Small birds, which are agile, are an important source of food for owls. Male burrowing owls have been observed to have longer wing chords than females, despite being smaller than females. Furthermore, owls have been observed to be the same size as their prey; this has been observed in other predatory birds, which suggests that owls with smaller bodies and long wing chords have been selected for because of the increased agility and speed that allows them to catch their prey.
Another popular theory suggests that females have not been selected to be smaller like male owls because of their sexual roles. In many species, female owls may not leave the nest. Therefore, females may have a larger mass to allow them to go for a longer period of time without starving. For example, one hypothesized sexual role is that larger females are more capable of dismembering prey and feeding it to their young, hence female owls are larger than their male counterparts. A different theory suggests that the size difference between male and females is due to sexual selection: since large females can choose their mate and may violently reject a male's sexual advances, smaller male owls that have the ability to escape unreceptive females are more to have been selected. All owls are carnivorous bi