Sex-specific trade-offs influence thermoregulation under climate change.

Increasingly, climate change is pushing species to the limits of their thermal tolerance, with cascading effects across ecosystems. Animals use behavior to prevent these harmful physiological states, but their need and ability to do so varies with their traits. Within species, traits such as sex and reproductive status affect heat sensitivity, perhaps eliciting differences in behavioral responses to thermal extremes. We evaluated whether sex and reproductive status affected thermoregulatory behavior and its efficacy in moose (Alces alces), a heat-sensitive endotherm that relies on thermal refuge. We expected traits associated with elevated heat load would be linked to heightened selection for thermal refuge and that differences in selection would successfully alleviate differing risks of overheating. Thus, reproductive females and males, who are more heat-sensitive, would have stronger selection for thermal refuge than non-reproductive females. We assessed selection of thermal refuge at bed sites and generated biophysical models to evaluate if selection mitigated risk of overheating. Reproductive status did not elicit differences in selection by females. The sexes, however, differed in selection of the trade-off between solar cover and cooling from wind. Females selected refuge with canopy cover and avoided wind. Males did not select cover and had weaker avoidance of wind than females. Yet, both sexes were more likely to overheat in areas of low cover, even if wind speeds were high. Hence, males had weaker selection of refuge than females despite being more likely to overheat, and life history trade-offs failed to explain the sub-optimal thermoregulatory behavior. We identify sex-specific thermoregulatory trade-offs, highlighting the disproportionate effects of climate change on certain demographic groups. Moreover, we emphasize the relevance of trait-based approaches for studying changing ecosystems.