Cold and alone? Roost choice and season affect torpor patterns in lesser short-tailed bats.

Abstract

Seasonal changes in weather and food availability differentially impact energy budgets of small mammals such as bats. While most thermal physiological research has focused on species that experience extreme seasonal temperature variations, knowledge is lacking from less variable temperate to subtropical climates. We quantified ambient temperature (T a) and skin temperature (T sk) responses by individuals from a population of New Zealand lesser short-tailed bats (Mystacina tuberculata) during summer and winter using temperature telemetry. During summer, communal roosts were more thermally stable than T a. During winter, solitary roosts were warmer than T a indicating significant thermal buffering. Communal roost trees were used on 83% of observation days during summer, and individuals occupying them rarely entered torpor. Solitary roosts were occupied on 93% of observation days during winter, and 100% of individuals occupying them used torpor. During summer and winter, bats employed torpor on 11 and 95% of observation days, respectively. Maximum torpor bout duration was 120.8h and winter torpor bout duration correlated negatively with mean T a. Torpor bout duration did not differ between sexes, although female minimum T sk was significantly lower than males. The summer Heterothermy Index varied, and was also significantly affected by T a. Mean arousal time was correlated with sunset time and arousals occurred most frequently on significantly warmer evenings, which are likely associated with an increased probability of foraging success. We provide the first evidence that torpor is used flexibly throughout the year by M. tuberculata, demonstrating that roost choice and season impact torpor patterns. Our results add to the growing knowledge that even small changes in seasonal climate can have large effects on the energy balance of small mammals.