Temperature effects on metabolic rate and torpor in southern forest bats (Vespadelus regulus)

I measured the metabolic rate (MR) of four male southern forest bats (Vespadelus regulus; 5.5 g) exposed to a diurnal increase in air temperature (T(a)) from 13 to 26 degrees C, simulating conditions in natural tree roosts. Three bats remained in torpor throughout the day, despite the rise in T(a), whereas one bat aroused at a T(a) of 25.2 degrees C and was normothermic for 108 min until re-entering torpor when T(a) declined in the afternoon. All bats aroused shortly after lights off. Torpid MR increased exponentially with rising T(a), yet even at 26 degrees C remained only 16% of minimum resting MR at the same T(a). Rest-phase energy expenditure (12 h), including the estimated cost of an evening arousal, ranged from 0.62 to 1.23 kJ. Thus, torpor provides these small bats with an enormous reduction in energy consumption even at T(a) close to their thermoneutral zone.