Sex-specific behavioural responses of elk to spatial and temporal variation in the threat of wolf predation

We studied individual and herd level behavioural responses of elk to spatial and temporal variation in the risk of predation by wolves over three winters in the Upper Gallatin drainage, Montana, U.S.A. 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, or 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 think 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 antipredator behaviours carry substantial foraging costs, that bulls, because of their poorer body condition, are less able to pay these costs than cows, and that differences in ability to pay foraging costs probably explain sex-specific differences in antipredator behaviours.