Using thermoregulatory profiles to assess climate change vulnerability in an arboreal tropical bat: heterothermy may be a pre-adaptive advantage

ABSTRACT: Many tropical endotherms are already confronted by ambient temperatures (Ta) close to their body temperature (Tb) and risk severe hyperthermia due to global warming. Tropical fruit bats play a vital role in the ecosystem and their absence could have dire consequences for ecosystem health. Many fruit bats have exposed roosting habits that increase their vulnerability to heat stress. We investigated the thermoregulatory capacity of wild caught lesser dog-faced fruit bats Cynopterus brachyotis from the Island of Borneo; a heterothermic 32 g foliage-roosting generalist bat. We determined the effect of Ta (21-36°C) on metabolism, Tb and evaporative cooling. We also measured the Ta and relative humidity (RH) at capture sites. The bats displayed a seemingly narrow thermoneutral zone of 30 ± 1°C, a basal metabolic rate of 5.60 ± 0.26 W kg-1 (1.01 ± 0.05 ml O2 g-1 h-1) and a normothermic Tb of 32.5 ± 0.3°C; all much lower than expected. Evaporative cooling was only effective at Ta ≤ 31°C, above which heat storage became apparent. Bats typically entered torpor at Ta < 25°C and thermoconformed (i.e. allowed their Tb to fluctuate with Ta) at Ta > 32°C. The microclimate at capture sites remained cool (Ta = 24-25°C) and humid (RH > 90%). Our study supports the argument that tropical endotherms are susceptible to hyperthermia due to their low Tb. Further, we discuss the potential advantage of heterothermy in coping with short-term heatwaves. However, the future of these bats, and likely other low Tb species, depends primarily on their thermally buffered habitats, and decisive conservation action is required to protect thermal refugia.