Torpor energetics are related to the interaction between body mass and climate in bats of the family Vespertilionidae

Torpor is an adaptive strategy allowing heterothermic animals to cope with energy limitations. In birds and mammals, intrinsic and extrinsic factors, such as body mass and ambient temperature, are the main variables influencing torpor use. A theoretical model of the relationship between metabolic rate during torpor and ambient temperature has been proposed. Nevertheless, no empirical attempts have been made to assess the model predictions under different climates. Using open-flow respirometry, we evaluated the ambient temperature at which bats entered torpor and when torpid metabolic rate reached its minimum, the reduction in metabolic rate below basal values, and minimum torpid metabolic rate in 11 bat species of the family Vespertilionidae with different body mass from warm and cold climates. We included data on the minimum torpid metabolic rate of five species we retrieved from the literature. We tested the effects using mixed-effect phylogenetic models. All models showed a significant interaction between body mass and climate. Smaller bats went into torpor and reached minimum torpid metabolic rates at warmer temperatures, showed a higher reduction in the metabolic rate below basal values, and presented lower torpid metabolic rates than larger ones. The slopes of the models were different for bats from different climates. These results are likely explained by differences in body mass and the metabolic rate of bats, which may favor larger bats expressing torpor in colder sites and smaller bats in the warmer ones. Further studies to assess torpor use in bats from different climates are proposed.