Predicting the effects of milder winters on the energy and water balance of a warm‐temperate hibernator

Abstract At the foundation of a species' vulnerability to climate change are the linkages between its physiological capacity, microclimate exposure and behavioural decision‐making. In mild Australian winters, the large‐footed myotis ( Myotis macropus ) hibernates in thermally variable roosts and regularly arouses from torpor to forage on insects. This bat provides a tractable system to examine how changes in temperature interact with physiological capacity and behavioural flexibility to shape energy and water balance in warm‐temperate hibernators. We integrated data from temperature telemetry, laboratory respirometry and biophysical modelling to link the expression of physiological states in the wild (i.e. behavioural decisions to enter and arouse from torpor and forage) with their energetic and water costs across thermal environments (warm‐temperate, subtropical and tropical). Daily energy expenditure (DEE) predicted from respirometry data differed by less than 7% from estimates generated by the biophysical model, permitting the estimation of hibernation energetics from free‐ranging skin‐temperature data. DEE was sixfold greater on days that included an arousal from torpor, and 40%–70% greater for males because they exhibited a ~2 h longer euthermic duration than females. In our warm‐temperate study site, DEE increased with sunset temperature, and projecting this relationship to tropical conditions predicted DEE values 2.6–3.6 times lower than those expected in tropical climates. Across temperate, subtropical and tropical sites, +2°C and +4°C warming scenarios shifted mean energy expenditure beyond the 75th and 90th percentiles of their current DEE distribution, respectively, highlighting increasing energetic demands for hibernators in warmer regions under projected climate change scenarios and the importance of reliable foraging opportunities to maintain energy balance. Read the free Plain Language Summary for this article on the Journal blog.