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The yaks are members of the family Bovidae (oxen), order Artiodactyla, which also includes the domestic cattle and existing wild cattle species such as the aurochs, the gaur or seladang, and the koupray. The generally accepted species name for yak is Bos grunniens, and it seems to have an affinity to bison, which belong to the same genus Bos. Like some other Bos species the yak is a large, massive animal with stout limbs and a long tail, usually tufted at the tip. Wild yak males measure 10.5 ft (3.25 m) in head and body. The shoulder height is over 6.5 ft (2 m), and they weigh 1,800-2,200 lb (820-1,000 kg). Females are smaller, and weigh about one third as much.

Both sexes bear horns which are black and positioned quite far apart on each extremity of the top of the skull. In the male, they are larger, curving up and then down. They are approximately oval in cross-section. There are no suborbital, inguinal, or interdigital glands in yaks. The body is covered with long hair, blackish brown in color; it hangs down almost to the ground like a fringe around the lower part of the shoulders, the sides, the flanks, and the thighs. Wild yaks live on the Tibetan plateau where temperatures in winter may drop to -40°F (-40°C). The long, thick coat protects the yak from these frigid temperatures.

Domesticated yaks are smaller, have weaker horns, and may vary greatly in color: red, mottled brown, or black.

Wild yaks inhabit desolate steppes at up to 20,000 ft (6,100 m) above sea level. They are expert climbers, sure-footed and sturdy. During the relatively warmer months of August and September, wild yaks remain in high areas with permanent snow, but spend the rest of the year at lower elevations. They feed chiefly on grass, herbs, and lichens.

The females with young congregate in large herds up to 1,000 animals. The males live alone most of the year, or in groups of not more than 12. The bulls join the herd and fight for the females when mating season begins in September. Only during that time do wild yaks make a strange grunting sound, which domesticated yaks emit all year round. After a gestation period of nine months, a single calf is born, usually in June, with births occurring every other year. The newborn becomes independent after a year, and reaches full size at six to eight years of age. Maximum life span for the wild yak is estimated at 25 years. Domesticated yaks may give birth every year. Wild yaks are classified as endangered by IUCN-The World Conservation Union. They are officially protected in China, but their numbers have declined due to uncontrolled hunting.

Yaks were probably domesticated during the first millennium B.C., and now they are found in association with people in the high plateaus and mountains of Central Asia. They are docile and powerful, and they are surely the most useful of domesticated animals at elevation above 6,500 ft (2,000 m). The serve as mounts, beasts of burden, for milk and meat, and are also sheared for their wool. There are about 12 million domesticated yaks, but perhaps no more than a few hundred wild ones.

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