The yak is a long-haired bovid found throughout the Himalayan region of south Central Asia, the Tibetan Plateau and as far north as Mongolia and Russia. Most yaks are domesticated yaks, Bos grunniens. There is also a small, vulnerable population of wild yaks, Bos mutus. In the 1990s, a concerted effort was undertaken to help save the wild yak population.
Yaks belong to the genus Bos, and are therefore related to cattle. Mitochondrial DNA analyses to determine the evolutionary history of yaks have been somewhat ambiguous.
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 by Linnaeus in 1766, but this name is now generally only considered to refer to the domesticated form of the animal, with Bos mutus 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.
Except where the wild yak is considered as a subspecies of Bos grunniens, there are no recognised subspecies of yak.
Wild yaks are among the largest bovids and are second only to the gaur in shoulder height. They are also the largest native animal in their range. Wild yak adults stand about 1.6 to 2.2 m tall at the shoulder and weigh 305–1,000 kg. The head and body length is 2.5 to 3.3 m, not counting the tail of 60 to 100 cm. The females are about one-third the weight and are about 30% smaller in their linear dimensions when compared to bull wild yaks. Domesticated yaks are much smaller, males weighing 350 to 580 kg and females 225 to 255 kg.
Yaks are heavily built animals with a bulky frame, sturdy legs, and rounded cloven hooves. They are the only wild bovids of this size with extremely dense, long fur that hangs down lower than the belly. Wild yaks are generally dark, blackish to brown, in coloration. However, domestic yaks can be quite variable in color, often having patches of rusty brown and cream. They have small ears and a wide forehead, with smooth horns that are generally dark in colour. In males, the horns sweep out from the sides of the head, and then curve forward; they typically range from 48 to 99 cm in length. The horns of females are smaller, only 27 to 64 cm 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. Yaks are highly friendly in nature and can easily be trained. There has been very little documented aggression from yaks towards human beings, although mothers can be extremely protective of their young and will bluff charge if they feel threatened.
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 males, 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. Wild yaks typically have black or dark brown hair over most of the body, with a greyish muzzle, although some wild golden-brown individuals have been reported. Wild yaks with gold coloured hair, known as Wild Golden Yak is considered an endangered sub-species by China, with an estimated population of 170 left in the wild. Domesticated yaks have a wider 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.
To the casual observer, yaks are easily confused with Highland cattle, but upon closer examination, the physical differences are quite apparent. For example, Highland cattle are shaggy, rather than long-haired, and have straight backs and tufted tails.
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 do not thrive at lower altitudes, and begin to suffer from heat exhaustion above about 15 °C. 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 so as to extract more nutrients. Yak consume the equivalent of 1% of their body weight daily while cattle require 3% to maintain condition.
Contrary to popular belief, yak and their manure have little to no detectable odor when maintained appropriately in pastures or paddocks with adequate access to forage and water. Yak's wool is naturally odor resistant.
Yaks mate in the summer, typically between July and September, depending on the local environment. For the remainder of the year, many males wander in small bachelor groups away from the large herds, but, as the rut approaches, they become aggressive and regularly fight amongst each other to establish dominance. In addition to non-violent threat displays, bellowing, and scraping the ground with their horns, male yaks also compete more directly, repeatedly charging at each other with heads lowered or sparring with their horns. Like bison, but unlike cattle, males wallow in dry soil during the rut, often while scent-marking with urine or dung. Females enter oestrus up to four times a year, and females are receptive only for a few hours in each cycle.
Gestation lasts between 257 and 270 days, so that the young are born between May and June, and results in the birth of a single calf. The female finds a secluded spot to give birth, but the calf is able to walk within about ten minutes of birth, and the pair soon rejoin the herd. Females of both the wild and domestic forms typically give birth only once every other year, although more frequent births are possible if the food supply is good.
Calves are weaned at one year and become independent shortly thereafter. Wild calves are initially brown in colour, and only later develop the darker adult hair. Females generally give birth for the first time at three or four years of age, and reach their peak reproductive fitness at around six years. Yaks may live for more than twenty years in domestication or captivity, although it is likely that this may be somewhat shorter in the wild.
Yaks are herd animals. Herds can contain several hundred individuals, although many are much smaller. The herds consist primarily of females and their young, with a smaller number of adult males. The remaining males are either solitary, or found in much smaller groups, averaging around six individuals. Although they can become aggressive when defending young, or during the rut, wild yaks generally avoid humans, and may rapidly flee for great distances if any approach.
In parts of Tibet and Karakorum, yak racing is a form of entertainment at traditional festivals and is considered an important part of their culture. More recently, sports involving domesticated yaks, such as yak skiing, or yak polo, are being marketed as tourist attractions in Central Asian countries, including Gilgit–Baltistan, Pakistan.
In Nepal, Tibet and Mongolia, domestic cattle are crossbred with yaks. This gives rise to the infertile male dzo as well as fertile females known as dzomo or zhom, which may be crossed again with cattle. The "Dwarf Lulu" breed, "the only Bos primigenius taurus type of cattle in Nepal" has been tested for DNA markers and found to be a mixture of both taurine and zebu types of cattle with yak. According to the International Veterinary Information Service, the low productivity of second generation cattle-yak crosses makes them suitable only as meat animals.
Crosses between yaks and domestic cattle have been recorded in Chinese literature for at least 2,000 years. Successful crosses have also been recorded between yak and American bison, gaur, and banteng, generally with similar results to those produced with domestic cattle.