Torpor, thermal biology, and energetics in Australian long-eared bats (Nyctophilus)

Previous studies have suggested that Australian long-eared bats (Nyctophilu s) differ from northern-hemisphere bats with respect to their thermal physi ology and patterns of torpor. To determine whether this is a general trait of Australian bats, we characterised the temporal organisation of torpor an d quantified metabolic rates and body temperatures of normothermic and torp id Australian bats (Nyctophilus geoffroyi, 7 g and N. gouldi, 10 g) over a range of air temperatures and in different seasons. The basal metabolic rat e of normothermic bats was 1.36 +/- 0.17 mi g(-1) h(-1) (N. geoffroyi) and 1.22 +/- 0.13 mi g(-1) h(-1) (N. gouldi), about 65% of that predicted by al lometric equations, and the corresponding body temperature was about 36 deg rees C. Below an air temperature of about 25 degrees C bats usually remaine d normothermic for only brief periods and typically entered torpor. Arousal from torpor usually occurred shortly after the beginning of the dark phase and torpor re-entry occurred almost always during the dark phase after nor mothermic periods of only 111 +/- 48 min (N. geoffroyi) and 115 +/- 66 min (N. gouldi). At air temperatures below 10 OC, bats remained torpid for more than 1 day. Bats that were measured overnight had steady-state torpor meta bolic rates representing only 2.7% (N. geoffroyi) and 4.2% (N, gouldi) of t he basal metabolic rate, and their body temperatures fell to minima of 1.4 and 2.3 degrees C, respectively. In contrast, bats measured entirely during the day, as in previous studies, had torpor metabolic rates that were up t o ten times higher than those measured overnight. The steady-state torpor m etabolic rate of thermoconforming torpid bats showed an exponential relatio nship with body temperature (r(2) = 0.94), suggesting that temperature effe cts are important for reduction of metabolic rate below basal levels. Howev er, the 75% reduction of metabolic rate between basal metabolic rate and to rpor metabolic rate at a body temperature of 29.3 degrees C suggests that m etabolic inhibition also plays an important role. Torpor metabolic rate sho wed little or no seasonal change. Our study suggests that Australian Nyctop hilus bats have a low basal metabolic rate and that their patterns of torpo r are similar to those measured in bats from the northern hemisphere. The l ow basal metabolic rate and the high proclivity of these bats for using tor por suggest that they are constrained by limited energy availability and th at heterothermy plays a key role in their natural biology.