Thermal energetics of female big brown bats ('Eptesicus fuscus')

We investigated thermoregulation and energetics in female big brown bats, 'Eptesicus fuscus' (Beauvois, 1796). We exposed bats to a range of ambient temperatures (Ta) and used open-flow respirometry to record their metabolic responses. The bats were typically thermoconforming and almost always entered torpor at Tas below the lower critical temperature Tlc of 26.7 °C. Basal metabolic rate (BMR, 16.98 ± 2.04 mL O·₂h⁻¹, mean body mass = 15.0 ± 1.4 g) and torpid metabolic rate (TMR, 0.460 ± 0.07 mL O₂·h⁻¹, mean body mass = 14.7 ± 1.3 g) were similar to values reported for other vespertilionid bats of similar size and similar to a value for E. fuscus BMR calculated from data in a previous paper. However, we found that big brown bats had a lower Tlc and lower thermal conductance at low Ta relative to those measured in the previous study. During torpor, the minimum individual body temperature (Tb) that we recorded was 1.1 °C and the bats began defending minimum Tb at Ta of approximately 0 °C. BMR of big brown bats was 76% of that predicted for bats based on the relationship between BMR and body mass. However, the Vespert ilionidae have been under-represented in previous analyses of the relationship between BMR and body mass in bats. Our data, combined with data for other vespertilionids, suggest that the family may be characterized by a lower BMR than that predicted based on data from other groups of bats.