Why Some Caribou Migrate and Others Stay: Winter Strategies in the Western Arctic Herd

Not all caribou migrate, even within the same herd: some save energy, while others travel far to feed on lichens. NPS photo by Matt Cameron.

Two ways through winter

Migration is one way animals survive seasons of scarcity. Arctic caribou travel long distances south to winter ranges in fall and back north in spring. But not every caribou in a herd migrates every year — even in the Western Arctic Herd, one of North America's largest. Migrants may travel up to 200 miles farther in fall than those that stay, yet little was known about why some go and others don't, or whether the two groups behave differently in winter.

A team from the National Park Service and the University of Alaska Fairbanks tracked the winter movements of female caribou from the herd to find out.

What they found

• More food for migrants: caribou that migrated had more than 2.5 times as much lichen cover around them as those that stayed. Since caribou eat lichens almost exclusively in winter, better access to lichen in the south looks like a big reason to make the trip.
• Two strategies: migrants moved nearly twice as much in winter, seemingly searching out and eating as much lichen as possible — relying on income (food). Caribou that stayed moved less, relying more on capital — their body reserves.
• Slowing down to eat: both groups slowed down where lichen was thicker, likely spending time digging (cratering) through snow to reach it.
• Near villages: caribou moved a little more near villages, perhaps from disturbance, but the effect was weak.
• Saving energy: winter is when caribou move least all year. The energy saved by moving less was modest — but equal to 11% of the daily energy their bodies lose, or 47% of the demands of early pregnancy, which could affect a female's condition or her calf's growth unless she eats more to make up for it. When winter is the longest season, every calorie counts.

Sources

Based on "Caribou in the same herd have different strategies for winter," National Park Service, summarizing K. Joly, M. D. Cameron and R. G. White, "Behavioral adaptation to seasonal resource scarcity by Caribou (Rangifer tarandus) and its role in partial migration," Journal of Mammalogy; the Park Service's summary is a work of the United States government in the public domain, and the study's abstract is paraphrased; rewritten in hubnx's own words.