Temporal Responses to Warming: Do Wild Herbivores Trade Off Heat, Predators, and Humans?

Ongoing environmental changes are affecting behavioral responses of animal populations. Both warming temperatures and increased human disturbance may trigger adjustments in mammal activity patterns, for example, favoring activity switch to nighttime despite a greater risk of encountering nocturnal predators. Disentangling the relative roles of these stressors is critical for predicting the population-level consequences of environmental changes, yet the joint effect of multiple stressors is poorly understood. Here we investigated how ambient summer temperature, predators, and human presence influenced temporal responses in two herbivorous mammals (the roe deer Capreolus capreolus and the fallow deer Dama dama) across Mediterranean protected areas. By conducting intensive camera trapping (∼12,400 trapping days; 196 sites), we evaluated changes in daily activity level and nocturnality of deer species. Both herbivores reduced their daily activity with warmer temperatures, emphasizing the need to minimize thermoregulatory costs, yet only roe deer increased nocturnality following diel warming. Conversely, nocturnality of the more heat-tolerant fallow deer was only affected by wolf (Canis lupus) visitation rate, although weakly, suggesting that fallow deer traded off heat avoidance with predator avoidance. We found neither reductions in daily activity levels nor an increase in nocturnality in response to higher human visitation rate, possibly depending on our relatively undisturbed protected areas (i.e., areas with low human population density and sustainable levels of outdoor recreational activities) or the stronger effect of heat avoidance. Under the anticipated warming, species-specific consequences of these behavioral responses on population viability may be expected.