Yak

Appearance

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. 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. They have small ears and wide foreheads, 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. Males weigh 350 to 585 kg, females weigh 225 to 255 kg. Wild yaks can be substantially heavier, bulls reaching weights of up to 1,000 kilograms. Depending on the breed, domestic yak males are 111–138 centimetres high at the withers, while females are 105–117 centimetres 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 sound of cattle, but both wild and domestic yaks grunt and squeak, which inspired the scientific name of the domestic yak variant, "Bos grunniens". Nikolay Przhevalsky named the wild variant "Bos mutus" believing that it did not make a sound at all, but it does.

Naming

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", Tibetan: འབྲི་, Wylie: dri" or Tibetan: གནག, Wylie: "g.nag" 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.

Reproduction

Yaks mate in the summer, typically between July and September, depending on the local environment. For the remainder of the year, many bulls wander in small bachelor groups away from the large herds, but, as the rut approaches, they become aggressive and regularly fight among each other to establish dominance. In addition to non-violent threat displays, bellowing, and scraping the ground with their horns, bull 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 cow 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 color, 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.In Nepal, Tibet and Mongolia, domestic cattle are crossbred with yaks. This gives rise to the infertile male dzo མཛོ། as well as fertile females known as མཛོ་མོ། or, 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.

Evolution

Yaks mate in the summer, typically between July and September, depending on the local environment. For the remainder of the year, many bulls wander in small bachelor groups away from the large herds, but, as the rut approaches, they become aggressive and regularly fight among each other to establish dominance. In addition to non-violent threat displays, bellowing, and scraping the ground with their horns, bull 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 cow 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 color, 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.