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

Abstract

SUMMARYDaily patterns of body temperature (T b ) and energy expenditure in heterothermic endotherms are affected by changes in ambient temperature (T a ) and selection of suitable microclimates, yet most laboratory studies employ constant T a 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 T a 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 T a and passive rewarming from time of the day, we shifted the heating phase to commence at 06:00 h, 09:00 h or 12:00 h on each day. Bats entered torpor overnight and aroused the next day at a time corresponding to rising T a and passive rewarming. The critical T a (and torpid T b ) 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 T a 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 T a , indicating a trade-off between the benefits and energetic costs of normothermia during resting. Our experiment clearly shows that a daily T a 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.

Physiological flexibility and acclimation to food shortage in a heterothermic primate

“…In nature, grey mouse lemurs reach body temperatures as low as 11.3±1.8°C for an ambient temperature of 9.4°C (Schmid, 2000). Also, in the field, arousal should be helped by rising ambient temperature during the day, with a passive rewarming of torpid animals (Turbill et al, 2008;Schmid, 2000). Such energetic benefits driven by ambient temperature variations were prevented in our experimental design.…”

Are torpid bats immune to anthropogenic noise?

“…dusk), which suggests that the time of day affects the response of torpid bats to noise disturbance. In general, small nocturnal mammals, including many bat species, exhibit a strong propensity for torpor in the early morning, when daily T a is lowest (Körtner and Geiser, 2000;Turbill et al, 2008). Moreover, torpid animals become more sensitive to a variety of external stimulations as the bout of torpor progresses (Lyman et al, 1982).…”

“…As a result, tests conducted in the morning are more likely to occur in the early phases of torpor, when torpid animals are generally less sensitive to disturbances (Kristoffersson and Soivio, 1964;Twente and Twente, 1968). Turbill et al (Turbill et al, 2008) have provided clear evidence that the time of day affects the critical arousal temperature of torpid bats. The critical arousal temperature was lower when external heating occurred later in the day.…”

Friends with benefits: the role of huddling in mixed groups of torpid and normothermic animals

“…However, only arousals that were closer to midday coincided with a second arousal of nest mates, whereas the rewarming of a group member in the early morning did not obviously affect the T b of other animals. As many species arouse from torpor around midday, synchronized arousals might not be caused by disturbance but by circadian rhythms (Heller and Ruby, 2004;Turbill et al, 2008). However, our study also showed that cooling rates were negatively correlated with the number of warm, normothermic individuals in the group and that TBDs were shorter and T b,min higher the more individuals were normothermic.…”