Behavioral responses of elk (Cervus elaphus) to the threat of wolf (Canus lupus) predation

Abstract

We studied individual and herd level behavioral responses of elk to spatial and temporal variation in the risk of predation by wolves over three winters in the Upper Gallatin drainage, Montana. Within a given drainage, elk of both sexes moved into or closer to protective cover (timber) in response to wolf presence. Cow elk responded to elevated risk by increasing vigilance in exchange for foraging, and large mixed (cow, calf, spike) herds substantially decreased in size. In contrast, when wolves were present, bulls did not increase vigilance levels, nor decrease feeding, and small bull-only groups slightly increased in size. As a consequence, small bull-only herds and large mixed sex herds converged on a similar size when wolves were present.

We believe this response is a balancing of the benefits of risk dilution with increased detectability or attractiveness of larger herds to wolves. Based on proportions in the population, wolves overselected bulls and underselected cows as prey. Thus, bulls showed weaker antipredator responses than cows, despite facing a greater risk of predation. Using marrow fat content from elk killed by wolves as an indicator of body condition, bulls were in significantly worse body condition than cows throughout the winter, and condition deteriorated for both sexes as winter progressed. Overall, we conclude that: anti-predator behaviors carry substantial foraging costs; bulls, due to their poorer body condition, are less able to pay these costs than cows; and differences in ability to pay foraging costs likely explain sex specific differences in anti-predator behaviors.