Caribou Memory: Why the Western Arctic Herd Changed Where It Winters

The view from a caribou's video collar. Caribou decide whether to cross the Kobuk River and winter to the south based on memories of the previous winter. NPS.

A migration with a choice

Caribou of the Western Arctic Herd make some of the longest land migrations on Earth, across a vast area of northwestern Alaska. Their summers follow a pattern — they return to the same general calving area each year — but where they spend winter can change a great deal from year to year.

Park Service biologists, with researchers from the State University of New York and the University of Maryland, studied how caribou choose a winter range, looking at deaths from 2009 to 2020: how likely a caribou was to die each year, and when in the year.

What changed in 2016

• Survival fell: the herd has declined over the past decade, and since 2016 caribou have had lower survival — they aren't living as long.
• The deadly season shifted: before 2016, caribou were most likely to die in early summer (calving and insect season); since then, in winter and early spring.
• The winter range moved: before 2016, 75% of collared caribou crossed the Kobuk River — which divides their range — to winter south of it. Since 2016, only 38% have crossed; most now stay north.

Memory drives the choice

The surprise: whether caribou wintered north or south in a given year depended on how caribou that went south fared the winter before.

• If southern-wintering caribou had survived well, more went south the next winter.
• If many had died, fewer went south.

So the big shift in wintering areas since 2016 has been adaptive — good for the herd. The herd is still declining, but caribou appear to adjust where they winter based on the herd's memory of conditions, making the best of a bad situation. With the Arctic warming faster than anywhere else on the planet, this is one way these animals are coping.

From the study

The published study adds detail:

• The winter range shifted by more than 500 km, as caribou moved between 2012 and 2020 from reliably wintering on maritime tundra in the far southwest of their range to more often wintering in mountains to the east.
• It drew on nearly 200 documented deaths.
• In the maritime tundra, overwinter survival had routinely topped 95% before the shift, but fell to around 80% even as fewer caribou wintered there. In the mountains, survival was intermediate and steadier from year to year.
• So the shift only partly offset rising mortality, and seasonal survival patterns point to poorer nutrition in winter.
• The single strongest predictor of whether a caribou migrated south was survival in the south the previous winter — evidence of collective memory and decision-making helping a highly mobile species in a changing environment.

Read the study: Gurarie, E., C. Beaupré, O. Couriot, M. D. Cameron, W. F. Fagan and K. Joly, 2024, "Evidence for an adaptive, large-scale range shift in a long-distance terrestrial migrant," Global Change Biology 30(11): e17589.

Sources

Based on "Memory influences where Western Arctic Herd caribou spend the winter," Bering Land Bridge National Preserve, Cape Krusenstern National Monument, Gates of the Arctic National Park and Preserve, Kobuk Valley National Park and Noatak National Preserve, National Park Service, which quotes the study's abstract; a work of the United States government in the public domain. The source says that after 2016 "only 38% of the animals crossed the Kobuk River and remained to the north"; this page reads it as 38% crossing, the rest staying north. The study's graphical abstract, published under a non-commercial license, is not reproduced.