Responses of elk herd size to fine-scale spatial and temporal variation in the risk of predation by wolves

Many studies have examined grouping as a form of antipredator behaviour, but relatively few studies have examined how group size responds to natural variation in predation risk across space and through time. We studied the responses of elk, Cervus elaphus, herd size and composition to natural variation in the risk of predation by wolves, Canis lupus, in the Gallatin Canyon of Montana. We found that elk herd size increased as distance to protective cover increased. A positive association between group size and distance to cover is often interpreted as evidence that grouping is an antipredator response. However, we found that herd size increased only on days that wolves were absent. When wolves were present, herd sizes remained small at all distances from cover. This suggests that aggregation far from cover on days that wolves were absent was a foraging response, rather than an antipredator response. These data highlight interaction between temporal and spatial variation in predation risk, and suggest caution in conclusions about the antipredator benefits of grouping in the absence of direct data on risk or predator presence.