PHOTOGRAPH BY J. BERGER, WCS
Published June 19, 2014
Female wild yaks don't worry about a glass ceiling.
In fact, according to a new study led by the Wildlife Conservation Society, mother yaks leave males behind in the valley bottoms and forage above them on steep mountain slopes that average 15,994 feet.
That's a height at which humans would be likely to develop altitude sickness, but it suits the huge, grazing mothers and other females—which gather at similar heights but on less steep slopes, and tend to make up much larger groups than males—just fine.
Bigger than bison, wild adult yaks range in shoulder height from about 5 to 7 feet (1.5 to 2.1 meters) and in weight from about 670 to 2,200 pounds (304 to 998 kilograms).
The steep terrain probably helps protect calves from predators, and the more humid high meadows produce higher protein food, says Joel Berger, lead author of the study conducted by U.S. and Chinese researchers in the last two months of 2012.
Because yaks are even harder to track in temperate seasons, the team worked in rough conditions.
"It was late winter and pretty cold," says Berger, a member of the Wildlife Conservation Society, a professor of wildlife conservation at the University of Montana, and a National Geographic grantee. "And we were camping. I think the coldest was minus 24 degrees."
The researchers also crossed large stretches of what Berger called "polarlike desert," looking for the widely distributed animals.
The males, especially, are widely distributed. They tend to congregate no more than two at a time, while females tend to cluster in groups of 30 on average.
Past Is Prologue?
Yaks are the largest grazers north of the tropics—and larger than American bison, their close relatives. Like bison in the U.S., the yaks that inhabit the Tibetan-Qinghai Plateau were once threatened by hunting.
Today they're listed as endangered species by both the Chinese and American governments. About 20 years ago, before the strong intervention of the Chinese government, "one could see yak skulls spread across the high elevation steppes just as bison skulls were 150 years ago" in the U.S., says Berger.
Yaks and bison are similar to each other in many ways but also different. For instance, noting the types of habitats in which bison are found—ranging from Mexico's Chihuahuan Desert to the evergreen forests in the far north of Canada—and the yaks' restriction to the cold climes of Asia, the study notes that "it is unclear whether the ecological partitioning of space and time in yak males and females would parallel that for bison."
In contrast to bison, which have recovered to some extent, yaks—because of their remote habitat—afford conservationists an opportunity to study a species' adaptations to climate and biological challenges without having to factor in the impact of human activity and habitat fragmentation.
Knowing how yaks use high-elevation habitat could help strengthen conservation programs in the Tibetan-Himalayan region, where glacial melting is among the fastest on Earth. The study is centered in an area about the size of Montana and Nebraska together, which is covered by other large reserves.
Whether the threat is melting glaciers, poaching, or hybridization with domestic yaks, the study concludes, knowing whether—and how—these factors force change among yaks "will require knowing something about how the two genders use space."
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