Thermal physiology of pregnant and lactating female and male long-eared bats, Nyctophilus geoffroyi and N. gouldi

During roosting in summer, reproductive female bats appear to use torpor less frequently and at higher body temperatures (T_b) than male bats, ostensibly to maximise offspring growth. To test whether field observations result from differences in thermal physiology or behavioural thermoregulation during roosting, we measured the thermoregulatory response and energetics of captive pregnant and lactating female and male long-eared bats (Nyctophilus geoffroyi 8.9 g and N. gouldi 11.5 g) during overnight exposure to a constant ambient temperature (T_a) of 15°C. Bats were captured 1-1.5 h after sunset and measurements began at 21:22±0:36 h. All N. geoffroyi entered torpor commencing at 23:47 ± 01:01 h. For N. gouldi, 10/10 males, 9/10 pregnant females and 7/8 lactating females entered torpor commencing at 01: 10 ± 01:40 h. The minimum T_b of torpid bats was 15.6 ± 1.1°C and torpid metabolic rate (TMR) was reduced to 0.05 ± 0.02 ml O₂ g^-1 h^-1. Sex or reproductive condition of either species did not affect the timing of entry into torpor (F = 1.5, df = 2, 19, P = 0.24), minimum TMR (F = 0.21, df = 4, 40, P = 0.93) or minimum T_b (F = 0.76, df = 5, 41, P = 0.58). Moreover, sex or reproductive condition did not affect the allometric relationship between minimum resting metabolic rate and body mass (F = 1.1, df = 4, 37, P = 0.37). Our study shows that under identical thermal conditions, thermal physiology of pregnant and lactating female and male bats are indistinguishable. This suggests that the observed reluctance by reproductive females to enter torpor in the field is predominantly because of ecological rather than physiological differences, which reflect the fact that females roost gregariously whereas male bats typically roost solitarily.