A mechanistic model of endotherm hibernation applied to the endangered mountain pygmy possum under climate change.

Hibernation is an important strategy used by many endotherms to conserve energy and water. Global warming is changing species' phenology and hibernation patterns, but whether such changes are beneficial or harmful depends on the species' life history traits, physiology, morphology, and behavior. Mechanistic niche models can be used to make strong inferences on such responses by explicitly quantifying the consequences of changed hibernation patterns for energy and water requirements. However, they have yet to be adapted to heterothermic species. Here, we address this problem by extending the endotherm biophysical model of the NicheMapR package to encompass torpor. This model accurately predicts the energy requirements of hibernating mammals over a broad size range from microbats to bears. We then used this approach to assess the effect of climate change on a Critically Endangered hibernator, the Australian mountain pygmy possum (Burramys parvus). Specifically, we contrasted conditions for the year 2010 with two future climate-change scenarios (2or 4°C of average warming) to identify: (i) the projected changes in energy and water requirements; (ii) the advantage conferred by hibernating for the species' energy and water requirements; and (iii) the areas across southeastern Australia that could continue to support hibernation. We projected an 11%-43% reduction in hibernation hours for the mountain pygmy possum under our two climate-change scenarios. In consequence, requirements for energy increased by 4%-21%, and for water by 10%-34%. Under current conditions, hibernation reduces annual energy requirements by 44%-52% and annual water requirements by 32%-42%, but in our projections, this energetic and hydric benefit of hibernation will decline due to climate change. The total area where hibernating and not hibernating is energetically equivalent is projected to increase by 60% under 4°C warming, preventing recovery from the species' severely restricted distribution at present. Our results show that climate change will have a profound impact on the duration and patterns of hibernation, a key survival strategy, for Burramys. Our framework for analyzing changing hibernation patterns provides a new and general way to test the vulnerability and plasticity of hibernating endotherms under global change.