Memory influences where Western Arctic Herd caribou spend the winter

Caribou decide whether to cross the Kobuk River and winter to the south or to stay north of the river based on their memories of conditions from the previous winter.

Western Arctic Herd caribou have some of the longest terrestrial migrations on the planet, moving across an immense area of northwestern Alaska throughout their annual journey. While their summer habits have some consistency, such as returning to the same general calving area each year, where they spend the winter can be very different from year to year. How caribou choose their winter range was the focus of recent research by NPS biologists and researchers from State University of New York and the University of Maryland. The team looked at mortality events from 2009 to 2020, the likelihood a caribou would die each year, and the seasonal patterns of when caribou died within the year.

They found that while the herd has been in a general decline over the last decade, caribou have had lower survival rates since 2016 as compared to before that (so caribou aren’t living as long in recent years). The season in which caribou mortality is greatest has also changed. Before 2016, caribou had the greatest chance of dying in early summer (calving and insect harassment seasons) but since 2016, caribou had the greatest chance of dying in winter and early spring. 2016 also marks a major change in where Western Arctic Herd caribou were spending the winter. Before 2016, 75% of collared animals crossed the Kobuk River (a major river dividing the annual range from east to west) to spend the winter south of the river. After 2016, only 38% of the animals crossed the Kobuk River and remained to the north. The researchers were surprised to find that the choice of whether to winter north or south of the Kobuk River in a given year was related to how animals fared that migrated south the previous winter. If animals that went south had higher survival the previous winter, then animals were more likely to go south the following winter, and in contrast, if animals that went south had higher mortality the previous winter, then animals were less likely to go south the next winter.

These results reveal that the large change in wintering areas since 2016 has been adaptive (i.e., beneficial) for the herd. Even though the herd continues to decline, these results suggest that caribou adjust where they spend the winter based on memories of conditions the herd experiences, potentially making the best of a bad situation. The Arctic is warming faster than anywhere else on the planet and these findings reveal some of the ways these remarkable animals are coping with these changes.

The choice of whether to winter north or south of the Kobuk River in a given year is related to how animals fared that migrated south the previous winter. If animals that went south had higher survival the previous winter, then they were more likely to go south the following winter. The change in wintering areas since 2016 has been adaptive (beneficial) for the herd. Even though the herd continues to decline, these results suggest that caribou adjust where to winter, based on the best options.

Evidence for an adaptive, large-scale range shift in a long-distance terrestrial migrant

Abstract

Long-distance migrations are a striking, and strikingly successful, adaptation for highly mobile terrestrial animals in seasonal environments. However, it remains an open question whether migratory animals are more resilient or less resilient to rapidly changing environments. Furthermore, the mechanisms by which animals adapt or modify their migrations are poorly understood. We describe a dramatic shift of over 500 km in the wintering range of the Western Arctic Herd, a large caribou (Rangifer tarandus) herd in northwestern Alaska, an area that is undergoing some of the most rapid warming on Earth. Between 2012 and 2020, caribou switched from reliably wintering in maritime tundra in the southwestern most portion of their range to more frequently wintering in mountainous areas to the east. Analysis of this range shift, in conjunction with nearly 200 documented mortality events, revealed that it was both broadly adaptive and likely driven by collective memory of poor winter conditions. Before the range shift, overwinter survival in the maritime tundra was high, routinely surpassing 95%, but falling to around 80% even as fewer animals wintered there. Meanwhile, in the increasingly used mountainous portion of the range, survival was intermediate and less variable across years compared to the extremes in the southern winter ranges. Thus, the shift only imperfectly mitigated overall increased mortality rates. The range shift has also been accompanied by changes in seasonal patterns of survival that are consistent with poorer nutritional intake in winter. Unexpectedly, the strongest single predictor of an individual's probability of migrating south was the overall survival of animals in the south in the preceding winter, suggesting that the range shift is in part driven by collective memory. Our results demonstrate the importance and use of collective decision making and memory for a highly mobile species for improving fitness outcomes in a dynamic, changing environment.

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.