Caribou group size related to seasonal life events

Caribou group sized are large during migration, especially in the fall.

Caribou are a highly social member of the deer family and often form large groups. However, there are surprisingly few studies about caribou group sizes and how they change over the course of a year. In a new study, daily and monthly group sizes were reported for the Nelchina Caribou Herd, located in east-central Alaska. Here’s what we found:

• Group sizes were smallest in the winter and late summer, likely in response to foraging conditions; caribou need to spread out further to find enough food.
• Groups sizes were small during calving as caribou attempt to space away from each other to make it harder for predators to find them.
• Post-calving group sizes increased as caribou formed nursey bands.
• Group sizes were largest during peak insect harassment, reaching 10,000 caribou (or 30% of the herd) in a single group, further highlighting the importance of biting insects to caribou ecology.
• Fall migration had larger groups of caribou than spring migration.

As group sizes are influenced by density-dependent factors, predation pressure, foraging conditions, herd size, and other factors, understanding how caribou group sizes change over time can help biologists understand the ecology of the species and how best to manage them.

Year-round variation in monthly and daily group size estimates of barren-ground caribou in east-central Alaska

Abstract

Sociality plays a major role in an array of factors critical to the ecology of many species, including sexual selection, migration, habitat selection, predation risk, and parasite infection. Group size is a fundamental metric used to assess sociality in many species. While barren-ground caribou (Rangifer tarandus) are highly social, there is little quantitative documentation of the patterns in their year-round group sizes. I report the daily and monthly average group sizes of the Nelchina Caribou Herd in east-central Alaska. Group sizes were small not only in winter and late summer but also during calving. Group sizes increased post-calving and reached their maximum during the peak of insect harassment in July. The largest single group observed was >30% of the entire herd. July was the only month where the average group size was more than 1% of the size of the herd. Group sizes were relatively large during migration but larger during fall migration than spring, which may reflect the importance of sociality during the rut and/or the dispersed nature of barren-ground caribou during winter.

Joly, K. 2025. Year-round variation in monthly and daily group size estimates of barren-ground caribou in east-central Alaska. Journal of Zoology. DOI: 10.1111/jzo.70058.