Reproduction and Life History
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.
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