Caribou in the same herd have different strategies for winter

Not all caribou migrate, even within the same herd. Some stay put in the winter and save their energy while others travel long distances to search out and feed on lichens.

NPS/Matt Cameron

Migration is one way animals survive seasons of scarcity. For caribou in the Arctic, this means animals travel long distances as part of their southward migration to winter ranges. Then they reverse direction and travel northward in the spring. You may be surprised to know, though, that not all caribou within a herd will migrate every year. This is true for the Western Arctic Herd, one of the largest herds in North America. Obviously, migratory caribou have to travel far longer distances in fall to reach their winter grounds than non-migratory caribou, and this difference can be up to 200 miles. However, less is known about why some caribou migrate when others don’t and if these two groups behave differently when winter sets in. To help improve our understanding of these questions, a team of National Park Service and University of Alaska – Fairbanks researchers studied the movements of female Western Arctic Herd caribou during the winter months. They found that during winter, migrants found more than 2.5 times more lichen than those caribou that did not migrate. Caribou eat lichen almost exclusively in winter, and this pattern suggests that greater access to lichen in the south is a big reason why caribou make the long migration. Caribou that migrated also moved about twice as much as those that didn’t during winter. The researchers think this means the migrant animals were trying to search out and eat as much lichens as possible, while the caribou that didn’t migrate appeared to be reducing the energy they had to use by moving less throughout the winter. One thing both types of caribou had in common was that they moved less when they found patches of lichen, likely spending time to crater (dig) through snow to reach them. The energy savings for the caribou that skipped migration were modest in absolute terms, but every calorie counts when winter is the longest season of the year.

Behavioral adaptation to seasonal resource scarcity by Caribou (Rangifer tarandus) and its role in partial migration

Abstract

Animals living in seasonal environments have adopted a wide array of tactics used to deal with seasonal resource scarcity. Many species migrate between habitats to reach areas where food resources are more plentiful as an attempt to address energetic demands through foraging. We assessed the winter behavioral adaptations of Caribou (Rangifer tarandus), a large ungulate inhabiting Arctic and sub-Arctic regions known for seasonal resource scarcity. Movement rates of Caribou are the lowest of the year during winter, revealing 1 mechanism individuals use to reduce energy expenditures. However, migratory individuals moved nearly twice as much as nonmigratory individuals during winter, suggesting that migratory individuals rely more upon income (forage), whereas nonmigratory individuals rely more upon capital (bodily reserves). Lichens are the primary winter forage for large, migratory herds of Caribou, and migratory individuals experienced more than 2.5 times greater lichen cover than nonmigratory individuals. We documented that both groups slowed their movement in areas of greater lichen cover, suggesting increased foraging time in these areas. Movement rates were greater near villages, which may be suggestive of disturbance, but the effect was weak. Overall energy saved by reduced movement rates was modest. However, energy savings were 11% of daily body energy lost or 47% of the demands of early pregnancy, which potentially could affect individual condition and/or fetal growth if not offset by increases in forage intake.

Joly, K., M. D. Cameron, and R. G. White. In press. Behavioral adaptation to seasonal resource scarcity by Caribou (Rangifer tarandus) and its role in partial migration. Journal of Mammalogy.