Flexible Employment of Torpor in Squirrel Gliders (Petaurus norfolcensis): An Adaptation to Unpredictable Climate?

Torpor is a highly effective response to counter various ecological and physiological bottlenecks in endotherms. In this study, we examined interrelations between thermoregulatory responses and key environmental variables in free-living squirrel gliders (Petaurus norfolcensis) in a habitat with drastic climatic and ecological changes across seasons. To this end, we measured body temperature (Tb) and heart rate (fH) simultaneously throughout the year using implanted data loggers. Squirrel gliders in our study experienced fluctuations in ambient temperature (Ta) between -4.0°C and 44.1°C and expressed torpor at different times during the year. In contrast to our expectations, torpor seemed to be employed flexibly, on demand, and most frequently in spring rather than during the coldest and/or hottest periods. Torpor bouts lasted, on average, about 5 h, and Tb during torpor dropped as low as 17.9°C. The fH during torpor decreased below 50 bpm, which is about one-third of the basal level. The ability to record fH alongside Tb enabled us to also report periods of low fH during thermoconforming hyperthermia at Ta's above 35°C that likely occurred to conserve energy and water. Our findings double the body size of Australian gliders for which data on torpor are available and advance our ecological understanding of the dynamics of torpor expression in wild mammals and of how animals cope with varying conditions. Moreover, they highlight that the flexibility of physiology and thermoregulatory responses are clearly more complex than previously thought