Timing of the daily temperature cycle affects the critical arousal temperature and energy expenditure of lesser long-eared bats

Daily patterns of body temperature (Tb) and energy expenditure in heterothermic endotherms are affected by changes in ambient temperature (Ta) and selection of suitable microclimates, yet most laboratory studies employ constant Ta to measure metabolic rates. In particular, exposure to a daily temperature cycle, even within rest shelters, may be important in timing of torpor and arousal and determining resting energy costs in wild animals. We tested how captive bats (Nyctophilus geoffroyi; 7 g) exposed to a diurnal Ta fluctuation (between 13°C and 27°C), similar to natural conditions in their summer tree roosts, adjusted the timing of daily arousals. To distinguish the effects of Ta and passive rewarming from time of the day, we shifted the heating phase to commence at 06:00h, 09:00h or 12:00h on each day. Bats entered torpor overnight and aroused the next day at a time corresponding to rising Ta and passive rewarming. The critical Ta (and torpid Tb) for arousal was not fixed, however, but was lower when heating occurred later in the rest phase, providing the first evidence that the critical arousal Ta is affected by time of the day. Bats re-entered torpor in response to cooling late in the afternoon, yet always aroused at lights off. A period of normothermic thermoregulation was therefore closely synchronised with maximum daily Ta, indicating a trade-off between the benefits and energetic costs of normothermia during resting. Our experiment clearly shows that a daily Ta cycle affects the thermoregulatory behaviour and energetics of these small bats. More generally, these results demonstrate the critical influence of behavioural decisions on the daily energy expenditure of small heterothermic mammals.