Thermobiology, energetics and activity patterns of the Eastern tube-nosed bat ('Nyctimene robinsoni') in the Australian tropics: effect of temperature and lunar cycle

Currently, there are no data on the thermal biology of free-ranging pteropodid bats (Chiroptera). Therefore, our aim was to investigate physiological and behavioural strategies employed by the fruit bat 'Nyctimene robinsoni' (body mass ∼50 g) in winter in tropical Northern Queensland in relation to ambient temperature (Ta) and the lunar cycle. Daily body temperature (Tb) fluctuations in free-ranging bats were measured via radio-telemetry and metabolic rate was measured in captivity via open-flow respirometry (Ta, 15–30°C). Free-ranging bats showed a significant 24 h circadian cycle in Tb, with the lowest Tb at the end of the rest phase just after sunset and the highest Tb at the end of the activity phase just before sunrise. Average daily core Tb ranged from 34.7±0.6 to 37.3±0.8°C (mean ± s.d.) over an average daily Ta range of 17.1±1.1 to 23.5±1.8°C. Tb never fell below 30°C but Tb was significantly reduced during the full moon period compared with that during the new moon period. Tb was correlated with Ta during the second half of the rest phase (P<0.001) but not during the active phase. Resting metabolic rate of bats was significantly affected by Ta (P<0.001, R²=0.856). Our results show that tube-nosed bats exhibit reduced Tb on moonlit nights when they reduce foraging activity, but during our study torpor was not expressed. The energy constraints experienced here by tube-nosed bats with relatively moderate Ta fluctuations, short commuting distances between roosting and feeding locations, and high availability of food were probably not substantial enough to require use of torpor.