Fear, forage and fawns: Reproductive status determines response of adult female deer to bear and wolf predation risks in Southeast Alaska

Background/Question/Methods

Behavior of animals can be strongly influenced by both fear and hunger as animals seek to optimize fitness outcomes. In a multi-predator, single ungulate prey system, female ungulates may face risk-forage trade-offs that vary by reproductive status, as different predators focus on females versus fawns. Here we focus on a bear-wolf-deer system in Southeast Alaska, where adult female deer face predation by wolves, while fawns face predation by bears. In such a system, non-reproductive and pregnant females must balance risk to self (i.e., wolf predation) with access to forage, while females with fawns must incorporate both risk to self and risk to offspring (i.e., black bear predation) into selection decisions. We captured and GPS collared 35 adult female deer, tracked parturition daily, and closely monitored females and offspring. To examine patterns in resource selection, we developed time-dependent step-selection functions (SSFs) for female deer, with predictive variables including resource selection functions (RSFs) for bears and wolves, as well as forage abundance and habitat variables. Cox proportional hazards regression was used to link variation in resource selection of reproductive females was predictive of fawn and female survival times and outcomes.

Results/Conclusions

Our results demonstrate that for female deer, sensitivity to competing risks and subsequent resource selection depend on reproductive status. Pregnant and non-pregnant deer selected for high-forage, high bear risk, and moderate wolf risk in early spring, but pregnant deer shifted to avoiding areas with high bear risk as parturition neared. Deer with live fawns prioritized survival of fawns, selecting areas with minimal bear risk and abundant forage, and showed a higher tolerance of wolf risk while fawns were young. Deer that had lost fawns returned to patterns of resource selection similar to those of pregnant and non-reproductive deer, although with higher movement rates, potentially due to changing plant phenology in late summer. Maternal resource selection predicted risk to fawns, which increased with greater overlap with bears, and decreased with calendar date and fawn age. Our results indicate that habitat requirements for successfully reproducing females can be quite different from those of non-successful or non-reproductive individuals, with selection patterns driven by predation risk to neonatal offspring and access to forage during this key life-history phase. Offspring survival is often the most influential vital rate in real-world ungulate population dynamics, indicating that studies of resource needs during reproduction are needed to understand and manage ungulates in multi-predator systems.