Sex-specific responses to forage and predation risk drive divergent patterns of summer space use and survival in mountain caribou

Animals have evolved to acquire resources in ways that minimize the risk of predation. This trade-off between resource acquisition and predation risk can vary across space and time, as well as among individuals, altering animal behaviors, space-use patterns, and, ultimately, fitness traits. Most studies of forage-risk trade-offs focus on female animals during the reproductive season, with relatively few evaluating how these trade-offs operate differently between the sexes and may shift over time, as well as the implications for behavioral and demographic outcomes. We had the unique opportunity to assess temporal variation in forage-risk trade-offs during summer (10 May-31 August) in both male and female northern mountain caribou ( Rangifer tarandus ) in Denali National Park and Preserve, Alaska. We analyzed GPS telemetry data collected on caribou from 2010 to 2018, along with concurrent data on forage conditions and wolf space use, using a space-time point process model. We found that females exhibited the greatest forage-risk trade-off during early summer when calves are vulnerable by reducing their use of habitats with preferred forage in areas with higher predation risk. However, this trade-off waned in late summer when females switched strategies to prioritize forage over safety. Males did not exhibit a clear trade-off, but prioritized forage and reduced their movement rates to minimize energy expenditure. Importantly, we found that the probability of mortality for males was twice that of females, suggesting these sex-specific behavioral strategies had significant fitness consequences. Sex-specific responses to predation risk also contributed to sexual segregation of caribou during early summer, highlighting the role predation can play in the distribution of different demographic groups of animals.