The yak (Bos grunniens) is a species of long-haired domesticated cattle found throughout the Himalayan region, the Tibetan Plateau, Tajikistan, the Pamir Mountains, and as far north as Mongolia and Siberia. It is descended from the wild yak (Bos mutus).
Etymology The English word Yak originates from the Tibetan: གཡག་, Wylie: g.yag. In Tibetan and Balti it refers only to the male of the species, the female being called Tibetan: འབྲི་, Wylie: 'bri or Tibetan: གནག, Wylie: gnag in Tibetan and Tibetan: ཧཡག་མོ་, Wylie: hYag-mo in Balti. In English, as in most other languages that have borrowed the word, yak is usually used for both sexes, with bull or cow referring to each sex separately.
Taxonomy
Belonging to the genus Bos, yaks are related to cattle and bison. Mitochondrial DNA analyses to determine the evolutionary history of yaks have been inconclusive. The yak may have diverged from cattle at any point between one and five million years ago, and there is some suggestion that it may be more closely related to bison than to the other members of its designated genus. Apparent close fossil relatives of the yak, such as Bos baikalensis, have been found in eastern Russia, suggesting a possible route by which yak-like ancestors of the modern American bison could have entered the Americas. The species was originally designated as Bos grunniens ("grunting ox") by Linnaeus in 1766. Still, this name is now generally considered to refer only to the domesticated form of the animal, with Bos mutus ("mute ox") being the preferred name for the wild species. Although some authors still consider the wild yak to be a subspecies, Bos grunniens mutus, the ICZN made an official ruling in 2003 permitting the use of the name Bos mutus for wild yaks, and this is now the more common usage. There are no recognised subspecies of yak except where the wild yak is considered a subspecies of Bos grunniens.
Physical characteristics
Yaks are heavily built animals with bulky frames, sturdy legs, rounded, cloven hooves, and extremely dense, long fur hanging lower than the belly. While wild yaks are generally dark, blackish to brown in colouration, domestic yaks can be quite variable, often having rusty brown and cream patches. They have small ears and broad foreheads, with smooth horns that are generally dark in colour. In males (bulls), the horns sweep out from the sides of the head and then curve backwards; they typically range from 48 to 99 cm (19 to 39 in) in length.
The horns of females (cows) are smaller, at 27 to 64 cm (11 to 25 in) in length, and have a more upright shape. Both sexes have a short neck with a pronounced hump over the shoulders, although this is larger and more visible in males. Males weigh 350 to 585 kg (772 to 1,290 lb), females weigh 225 to 255 kg (496 to 562 lb). Wild (feral) yaks can be substantially heavier, bulls reaching weights of up to 1,000 kg (2,200 lb). Depending on the breed, domestic yak males are 111–138 cm (44–54 in) high at the withers, while females are 105–117 cm (41–46 in) high at the withers. Both sexes have long, shaggy hair with a dense woolly undercoat over the chest, flanks, and thighs to insulate them from the cold. Especially in bulls, this may form a long "skirt" that can reach the ground. The tail is long and horselike rather than tufted like the tails of cattle or bison. Domesticated yaks have a wide range of coat colours, with some individuals being white, grey, brown, roan or piebald. The udder in females and the scrotum in males are small and hairy as protection against the cold. Females have four teats. Yaks are not known to produce the characteristic lowing (mooing) sound of cattle, but both wild and domestic yaks grunt and squeak, which inspired the scientific name of the domestic yak variant, Bos grunniens (grunting bull). Nikolay Przhevalsky named the wild variant Bos mutus (silent bull), believing that it did not make a sound at all, but it does.
Physiology
Yak physiology is well adapted to high altitudes, having larger lungs and heart than cattle found at lower altitudes, as well as greater capacity for transporting oxygen through their blood, due to the persistence of foetal haemoglobin throughout life. Conversely, yaks have trouble thriving at lower altitudes, and are prone to suffering from heat exhaustion above about 15 °C (59 °F). Further adaptations to the cold include a thick layer of subcutaneous fat and an almost complete lack of functional sweat glands. Compared with domestic cattle, the rumen of yaks is unusually large, relative to the omasum. This likely allows them to consume greater quantities of low-quality food at a time, and to ferment it longer to extract more nutrients. Yak consume the equivalent of 1% of their body weight daily while cattle require 3% to maintain condition. They are grazing herbivores, with their wild ancestors feeding primarily on grass and sedges, with some herbs and dwarf shrubs.
Reproduction and life history
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