Canidae
Canidae is a biological family of caniform carnivorans, constituting a clade. A member of this family is a canid, colloquially referred to as dogs. The family includes three subfamilies: the Caninae, and the extinct Borophaginae and Hesperocyoninae. The Caninae are the canines, and include domestic dogs, wolves, coyotes, raccoon dogs, foxes, jackals and other species.
Canids are found on all continents except Antarctica, having arrived independently or accompanied by human beings over extended periods of time. Canids vary in size from the gray wolf to the fennec fox. The body forms of canids are similar, typically having long muzzles, upright ears, teeth adapted for cracking bones and slicing flesh, long legs, and bushy tails. They are mostly social animals, living together in family units or small groups and behaving co-operatively. Typically, only the dominant pair in a group breeds and a litter of young are reared annually in an underground den. Canids communicate by scent signals and vocalizations. One canid, the domestic dog, originated from a symbiotic relationship with Upper Paleolithic humans and is one of the most widely kept domestic animals.
Taxonomy
In the history of the carnivores, the family Canidae is represented by the two extinct subfamilies designated as Hesperocyoninae and Borophaginae, and the extant subfamily Caninae. This subfamily includes all living canids and their most recent fossil relatives. All living canids as a group form a dental monophyletic relationship with the extinct borophagines, with both groups having a bicuspid on the lower carnassial talonid, which gives this tooth an additional ability in mastication. This, together with the development of a distinct entoconid cusp and the broadening of the talonid of the first lower molar, and the corresponding enlargement of the talon of the upper first molar and reduction of its parastyle distinguish these late Cenozoic canids and are the essential differences that identify their clade.The cat-like Feliformia and dog-like Caniformia emerged within the Carnivoramorpha around 45–42 Mya. The Canidae first appeared in North America during the Late Eocene. They did not reach Eurasia until the Late Miocene or to South America until the Late Pliocene.
Phylogenetic relationships
This cladogram shows the phylogenetic position of canids within Caniformia:Evolution
The Canidae are a diverse group of some 37 species ranging in size from the maned wolf with its long limbs to the short-legged bush dog. Modern canids inhabit forests, tundra, savannas, and deserts throughout tropical and temperate parts of the world. The evolutionary relationships between the species have been studied in the past using morphological approaches, but more recently, molecular studies have enabled the investigation of phylogenetics relationships. In some species, genetic divergence has been suppressed by the high level of gene flow between different populations and where the species have hybridized, large hybrid zones exist.Eocene epoch
ns evolved after the extinction of the non-avian dinosaurs 66 million years ago. Around 50 million years ago, or earlier, in the Paleocene, the Carnivora split into two main divisions: caniform and feliform. By 40 Mya, the first identifiable member of the dog family had arisen. Named Prohesperocyon wilsoni, its fossils have been found in southwest Texas. The chief features which identify it as a canid include the loss of the upper third molar, and the structure of the middle ear which has an enlarged bulla. Prohesperocyon probably had slightly longer limbs than its predecessors, and also had parallel and closely touching toes which differ markedly from the splayed arrangements of the digits in bears.Canidae soon divided into three subfamilies, each of which diverged during the Eocene: Hesperocyoninae, Borophaginae, and Caninae. Members of each subfamily showed an increase in body mass with time and some exhibited specialized hypercarnivorous diets that made them prone to extinction.
Oligocene epoch
By the Oligocene, all three subfamilies had appeared in the fossil record of North America. The earliest and most primitive branch of the Canidae was Hesperocyoninae, which included the coyote-sized Mesocyon of the Oligocene. These early canids probably evolved for the fast pursuit of prey in a grassland habitat; they resembled modern viverrids in appearance. Hesperocyonines eventually became extinct in the middle Miocene. One of the early Hesperocyonines, the genus Hesperocyon, gave rise to Archaeocyon and Leptocyon. These branches led to the borophagine and canine radiations.Miocene epoch
Around 8 Mya, the Beringian land bridge allowed members of the genus Eucyon a means to enter Asia from North America and they continued on to colonize Europe.Pliocene epoch
The Canis, Urocyon, and Vulpes genera developed from canids from North America, where the canine radiation began. The success of these canids was related to the development of lower carnassials that were capable of both mastication and shearing. Around 5 million years ago, some of the Old World Eucyon evolved into the first members of Canis, In the Pliocene, around 4–5 Mya, Canis lepophagus appeared in North America. This was small and sometimes coyote-like. Others were wolf-like. C. latrans is theorized to descend from C. lepophagus.The formation of the Isthmus of Panama, about 3 Mya, joined South America to North America, allowing canids to invade South America, where they diversified. However, the last common ancestor of the South American canids lived in North America some 4 Mya and more than one incursion across the new land bridge is likely given the fact that more than one lineage is present in South America. Two North American lineages found in South America are the gray fox and the now-extinct dire wolf. Besides these, there are species endemic to South America: the maned wolf, the short-eared dog, the bush dog, the crab-eating fox, and the South American foxes. The monophyly of this group has been established by molecular means.
Pleistocene epoch
During the Pleistocene, the North American wolf line appeared, with Canis edwardii, clearly identifiable as a wolf, and Canis rufus appeared, possibly a direct descendant of C. edwardii. Around 0.8 Mya, Canis ambrusteri emerged in North America. A large wolf, it was found all over North and Central America and was eventually supplanted by the dire wolf, which then spread into South America during the Late Pleistocene.By 0.3 Mya, a number of subspecies of the gray wolf had developed and had spread throughout Europe and northern Asia. The gray wolf colonized North America during the late Rancholabrean era across the Bering land bridge, with at least three separate invasions, with each one consisting of one or more different Eurasian gray wolf clades. MtDNA studies have shown that there are at least four extant C. lupus lineages. The dire wolf shared its habitat with the gray wolf, but became extinct in a large-scale extinction event that occurred around 11,500 years ago. It may have been more of a scavenger than a hunter; its molars appear to be adapted for crushing bones and it may have gone extinct as a result of the extinction of the large herbivorous animals on whose carcasses it relied.
In 2015, a study of mitochondrial genome sequences and whole-genome nuclear sequences of African and Eurasian canids indicated that extant wolf-like canids have colonized Africa from Eurasia at least five times throughout the Pliocene and Pleistocene, which is consistent with fossil evidence suggesting that much of African canid fauna diversity resulted from the immigration of Eurasian ancestors, likely coincident with Plio-Pleistocene climatic oscillations between arid and humid conditions. When comparing the African and Eurasian golden jackals, the study concluded that the African specimens represented a distinct monophyletic lineage that should be recognized as a separate species, Canis anthus. According to a phylogeny derived from nuclear sequences, the Eurasian golden jackal diverged from the wolf/coyote lineage 1.9 Mya, but the African golden wolf separated 1.3 Mya. Mitochondrial genome sequences indicated the Ethiopian wolf diverged from the wolf/coyote lineage slightly prior to that.Wild canids are native to all continents except Australasia and Antarctica, and also occur as feral in New Guinea and Australia. They inhabit a wide range of different habitats, including deserts, mountains, forests, and grasslands. They vary in size from the fennec fox, which may be as little as in length and weigh, to the gray wolf, which may be up to long, and can weigh up to. Only a few species are arboreal—the gray fox, the closely related island fox and the raccoon dog habitually climb trees.
All canids have a similar basic form, as exemplified by the gray wolf, although the relative length of muzzle, limbs, ears, and tail vary considerably between species. With the exceptions of the bush dog, the raccoon dog and some domestic dog breeds, canids have relatively long legs and lithe bodies, adapted for chasing prey. The tails are bushy and the length and quality of the pelage vary with the season. The muzzle portion of the skull is much more elongated than that of the cat family. The zygomatic arches are wide, there is a transverse lambdoidal ridge at the rear of the cranium and in some species, a sagittal crest running from front to back. The bony orbits around the eye never form a complete ring and the auditory bullae are smooth and rounded. Females have three to seven pairs of mammae.
All canids are digitigrade, meaning they walk on their toes. The tip of the nose is always naked, as are the cushioned pads on the soles of the feet. These latter consist of a single pad behind the tip of each toe and a more-or-less three-lobed central pad under the roots of the digits. Hairs grow between the pads and in the Arctic fox the sole of the foot is densely covered with hair at some times of the year. With the exception of the four-toed African wild dog, five toes are on the forefeet, but the pollex is reduced and does not reach the ground. On the hind feet are four toes, but in some domestic dogs, a fifth vestigial toe, known as a dewclaw, is sometimes present, but has no anatomical connection to the rest of the foot. In some species, slightly curved nails are non-retractile and more-or-less blunt while other species have sharper, partially-retractile claws.
The canine penis contains a baculum and a structure called the bulbus glandis that expands during copulation, forming a copulatory tie that lasts for up to an hour. Young canids are born blind, with their eyes opening a few weeks after birth. All living canids have a ligament analogous to the nuchal ligament of ungulates used to maintain the posture of the head and neck with little active muscle exertion; this ligament allows them to conserve energy while running long distances following scent trails with their nose to the ground. However, based on skeletal details of the neck, at least some of the Borophaginae are believed to have lacked this ligament.