Do season and distribution affect thermal energetics of a hibernating bat endemic to the tropics and subtropics?

Although many tropical and subtropical areas experience pronounced seasonal changes in weather and food availability, few studies have examined and none have compared the thermal physiology and energetics of a hibernating mammal that is restricted to these regions. We quantified thermal energetics of northern long-eared bats ('Nyctophilus bifax'; body mass ~10 g) during summer, winter, and spring from a subtropical habitat, and also during winter from a tropical habitat, to determine how 'N. bifax' cope with climate and seasonal changes in weather. We captured bats in the wild and measured metabolic rates via open-flow respirometry. The basal metabolic rate of subtropical bats at an ambient temperature (Ta) of 32.6 ± 0.7°C was 1.28 ± 0.06 ml O₂·g⁻¹·h⁻¹ during both summer and winter, similar to other species of 'Nyctophilus'. Resting metabolic rates below the thermoneutral zone increased similarly with decreasing Ta during all seasons and in both regions. All individuals showed a high proclivity to enter torpor at Ta values below the thermoneutral zone. Metabolic rates in torpid thermoconforming bats fell with Ta and body temperature, and mean minimum metabolic rates during torpor were similar during all seasons and in both regions and as predicted from body mass in temperate zone hibernators. At very low Ta, torpid 'N. bifax' thermoregulated, and this threshold Ta differed significantly between subtropical (Ta = 3.5 ± 0.3°C) and tropical (Ta = 6.7 ± 0.7°C) individuals, but not between seasons. Our data show that thermal energetics of 'N. bifax' do not vary seasonally and in many aspects are similar in tropical and subtropical bats; however, torpid individuals from the subtropics allow body temperature to fall to significantly lower values than those from the tropics.