Social organization of boreal woodland caribou (Rangifer tarandus caribou) in response to decreasing annual snow depth

Global climate warming is causing reductions in the depth and duration of snow cover across much of the distributional range of threatened boreal woodland caribou (Rangifer tarandus caribou). Plasticity in group size and composition among gregarious ungulates likely reflects a response to a variety of proximate factors, including environmental conditions such as snow depth. Yet, longitudinal studies of social organization by ungulates in response to environmental conditions are limited. We used 11 years of aerial survey data to examine winter group size in a northern population of boreal woodland caribou in response to late-winter snow depth. We documented caribou social organization (size and types of groups) and annual variability in group size. We tested the hypothesis that group size increases with increasing snow depth. Most groups (61%; n = 456) were ≤ 5 animals; however, they only constituted 37% of the animals. Caribou were distributed among four types of groups, including solitary females, males only, females only, and mixed groups. Group sizes differed among group types. Mixed groups were both the most common and largest groups encountered in late winter. We found a positive relationship between snow depth and group size; however, there was no relationship between snow depth and the percentage of mixed groups. These caribou may form larger groups in winters with deep snow as a behavioral strategy to reduce energetic costs, predation risk, or both. Climate-induced decreases in late-winter snow depth will likely reduce group sizes in boreal woodland caribou. However, further work is required to understand if this change in social organization will affect the persistence of local boreal woodland caribou populations.