# Influence of Migratory Strategy, Group Size, and Environmental Conditions on the Movements of Caribou in Eastern Alaska

## TL;DR

This study explores how migration patterns, group size, and weather influence caribou movements in Alaska, aiming to improve conservation efforts.

## Contribution

The study quantifies how migratory strategy, group size, and environmental conditions influence caribou movement rates across seasons.

## Key findings

- Caribou that migrate longer distances travel more annually and have higher movement rates in non-migratory months.
- Larger group sizes correlate with increased movement rates, possibly due to competition or herd dynamics.
- Warmer temperatures and shallower snow are linked to higher movement rates in caribou.

## Abstract

Many factors affect the movements of migratory animals. The objective of this study was to assess if migratory strategy (whether individuals migrated to distant winter ranges or remained close to their summer range), the number of other individuals present nearby (group size), and environmental conditions, like snow depth and temperature, affected the movements of caribou. Migratory caribou moved more than resident caribou even during non-migratory months. Caribou found in larger groups also had greater movement rates. Warmer temperatures and shallower snow were associated with greater movement rates. A better understanding of what drives movements of caribou will help improve the management of this globally declining species.

Migration is a diverse behavior exhibited by a wide array of organisms. Variability in the type of movements is rooted in their purpose, environmental factors, demographics, and individual physiological condition. The ability of caribou (Rangifer tarandus granti) to efficiently move long distances and have a high degree of plasticity in their movements allows them to respond and be resilient to dynamic and dramatic differences in environmental conditions. I used 88 collared, sympatric, adult, female, barren-ground Nelchina Caribou Herd caribou in east-central Alaska to assess their migratory strategy (as indexed by the distance between winter and summer ranges) and how this might affect their movements. Employing 41,682 movement vectors from 39 of these individuals equipped with GPS collars, I compared the annual and monthly movements of caribou that were found on different winter ranges. Distances between winter and summer ranges for individual caribou were correlated with their annual movement, but not for caribou that wintered within the same area. As expected, caribou with the greatest distance between their winter and summer ranges (300 km) traveled the most annually (2132 km/year), whereas caribou with the shortest distance between ranges (71 km) traveled the least annually (1368 km/year). However, caribou that migrated the furthest exhibited greater movement rates in all non-migratory summer months and most non-migratory winter months, as well as during migration. Movement rates were the greatest in summer, peaking in July, regardless of where caribou wintered. During the winter months, movement rates were similar among caribou found on different winter ranges and decreased over the winter, reaching minimums in January-March. Caribou that migrated the shortest distance and had lower movement rates tended to be found in smaller groups in summer. The connection between group size and movement rates may be a function of competition or a small-scale example of the larger-scale phenomenon of range expansion of large herds. Environmental factors, such as snow depth and temperature, were also correlated (negatively and positively, respectively) with caribou movement rates. Survival was not significantly different for caribou utilizing different winter ranges, which implies that the benefits of this long-distance migration can be offset by its costs. A more detailed understanding of the drivers and variability of caribou movement should help improve the management of this declining species.