Further adaptations to the cold include a thick layer of subcutaneous fat, and an almost complete lack of functional sweat glands. Conversely, yaks have trouble thriving at lower altitudes, and are prone to suffering from heat exhaustion above about 15 ☌ (59 ☏).
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. Yak rider near Tsomgo Lake, Sikkim (3700 m) Nikolay Przhevalsky named the wild variant Bos mutus (silent bull) believing that it did not make a sound at all, but it does. 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).
The udder in females and the scrotum in males are small and hairy, as protection against the cold. Domesticated yaks have a wide range of coat colours, with some individuals being white, grey, brown, roan or piebald. The tail is long and horselike rather than tufted like the tails of cattle or bison. Especially in bulls, this may form a long "skirt" that can reach the ground. īoth sexes have long shaggy hair with a dense woolly undercoat over the chest, flanks, and thighs to insulate them from the cold. Depending on the breed, domestic yak males are 111–138 centimetres (44–54 in) high at the withers, while females are 105–117 centimetres (41–46 in) high at the withers. Wild yaks can be substantially heavier, bulls reaching weights of up to 1,000 kilograms (2,200 lb). Both sexes have a short neck with a pronounced hump over the shoulders, although this is larger and more visible in males. The horns of females (cows) are smaller, only 27 to 64 cm (11 to 25 in) in length, and have a more upright shape. They typically range from 48 to 99 cm (19 to 39 in) in length. In males (bulls), the horns sweep out from the sides of the head, and then curve forward. They have small ears and wide foreheads, with smooth horns that are generally dark in colour. While wild yaks are generally dark, blackish to brown in colouration, domestic yaks can be quite variable in colour, often having patches of rusty brown and cream.
Yaks are heavily built animals with bulky frames, sturdy legs, rounded, cloven hooves, and extremely dense, long fur that hangs down lower than the belly. Įxcept where the wild yak is considered as a subspecies of Bos grunniens, there are no recognised subspecies of yak. 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. The species was originally designated as Bos grunniens ("grunting ox") by Linnaeus in 1766, but 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. 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 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. Mitochondrial DNA analyses to determine the evolutionary history of yaks have been inconclusive. Belonging to the genus Bos, Yaks are related to cattle ( Bos primigenius).
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