Captive Big Brown Bats ( Eptesicus fuscus ) Display Hypothermia and Hypometabolism

Abstract

Many temperate small mammals use heterothermy as a strategy to combat the energy costs of maintaining a high body temperature (Tb) during cold periods. Thus, heterothermy is a balancing act for small mammal energy conservation with the benefits of endothermy, and is variable depending on environmental factors such as ambient temperature (TA) and food availability. In temperate bat species, heterothermy is typically viewed as two distinct categories: torpid or euthermic. However, when a temperate bat undergoes acclimation to a new environment, such as captivity, it is possible for torpor and euthermia to become less distinct and instead reflect a range of Tb across TA. We hypothesized that captive big brown bats (Eptesicus fuscus) would have skin temperatures (Tskin) and metabolic rates at varying TA ranging between those of wild big brown bats in torpor and euthermia. To record captive metabolic rates by open flow respirometry, we used 11 big brown bats housed for > 6 years at Northeast Ohio Medical University (Rootstown, OH). We recorded metabolic rates at TA of 0, 5, 10, 15, 20, 25, 30, 35, and 37.5 ℃. For wild big brown bat comparisons, we extracted data for TA, Tb and metabolic rates from primary literature. We categorized wild bat Tb and metabolic rates as ‘torpid’ or ‘euthermic’ as indicated by authors within the literature. For captive bats, we labeled Tskin and metabolic rates as ‘unknown’ to eliminate our own bias on obscure values for statistical analyses. To determine differences across these categories, we created two separate linear models for 1) Tb and Tskin and 2) mass-specific metabolic rates as functions of an interaction between TA and thermoregulatory categories. We used analysis of variance and Tukey tests to determine if this interaction explained Tb and Tskin and mass-specific metabolic rates, and what thermoregulatory categories drove those effects, respectively. TA and thermoregulatory categories described differences between wild (‘torpid’ and ‘euthermic’) and captive (‘unknown’) big brown bats for Tb and Tskin (P < 0.0001) and mass-specific metabolic rates (P < 0.0001). Across all TA, wild ‘euthermic’ big brown bats had greater Tb than Tskin of captive ‘unknown’ bats (P < 0.0001), and captive ‘unknown’ bats had greater Tskin than Tb of wild ‘torpid’ bats (P < 0.0001). Similarly, wild ‘euthermic’ big brown bats had greater mass-specific metabolic rates than those of captive ‘unknown’ bats (P < 0.0001), and captive ‘unknown’ bats had greater mass-specific metabolic rates than wild ‘torpid’ bats (P < 0.0001). This suggests captive big brown bats experienced an intermediate hypothermia and hypometabolism compared with the torpor or euthermia of wild bats. Intermediate hypothermia and hypometabolism has not yet been described in primary literature in a temperate bat species. Future research will need to tease apart differences between general effects of captivity and temperature acclimation on captive big brown bats. Our work highlights that the spectrum of heterothermy in temperate small mammals extends to big brown bats and their variability in energy conservation strategies.