Environmental conditions, rather than season, determine torpor use and temperature selection in large mouse-eared bats (Myotis myotis)

We tested whether food availability, thermal environment and time of year affect torpor use and temperature selection in the large mouse-eared bat (Myotis myotis) in summer and winter. Food-deprived bats were torpid longer than bats offered food ad libitum. Bats placed in a gradient of low (0 °C–25 °C) ambient temperatures (Ta) spent more time in torpor than bats in a gradient of high (7 °C–43 °C) Ta's. However, we did not observe seasonal variations in the use of torpor. Moreover, even when food deprived in winter, bats never entered prolonged torpor at Ta's characteristic of their natural hibernation. Instead, bats preferred shallow torpor at relatively high Ta, but they always maintained a difference between body and ambient temperatures of less than 2 °C. Calculations based on respirometric measurements of metabolic rate showed that food deprived bats spent less energy per unit of time in torpor than fed individuals, even when they entered torpor at higher Ta's. We conclude that Ta likely serves as a signal of food availability and daily torpor is apparently an adaptation to unpredictable changes in food availability, such as its decrease in summer or its increase in winter. Thus, we interpret hibernation to be a second step in the evolution of heterothermy in bats, which allows survival in seasonal environments.