Can bats sense smoke during deep torpor?

While torpor is a beneficial energy-saving strategy, it may incur costs if an animal is unable to respond appropriately to external stimuli, which is particularly true when it is necessary to escape from threats such as fire. We aimed to determine whether torpid bats, which are potentially threatened because they must fly to escape, can sense smoke and whether respiration rate (RR), heart rate (HR) and reaction time of torpid bats prior to and following smoke introduction is temperature-dependent. To test this we quantified RR and HR of captive Australian tree-roosting bats, Nyctophilus gouldi (n = 5, ~ 10 g), in steady-state torpor in response to short-term exposure to smoke from Eucalyptus spp. leaves between ambient temperatures (Ta) of 11 and 23 °C. Bats at lower Ta took significantly longer (28-fold) to respond to smoke, indicated by a cessation of episodic breathing and a rapid increase in RR. Bats at lower Ta returned to torpor more swiftly following smoke exposure than bats at higher Ta. Interestingly, bats at Ta < 15 °C never returned to thermoconforming steady-state torpor prior to the end of the experimental day, whereas all bats at Ta ≥ 15 °C did, as indicated by apnoeic HR. This shows that although bats at lower Ta took longer to respond, they appear to maintain vigilance and prevent deep torpor after the first smoke exposure, likely to enable fast escape. Our study reveals that bats can respond to smoke stimuli while in deep torpor. These results are particularly vital within the framework of fire management conducted at Ta < 15 °C, as most management burns are undertaken during winter when bats will likely respond more slowly to fire cues such as smoke, delaying the time to escape from the fire.