Body temperatures of hibernating little brown bats reveal pronounced behavioural activity during deep torpor and suggest a fever response during white-nose syndrome

Abstract

Hibernating animals use torpor [reduced body temperature (Tb) and metabolic rate] to reduce energy expenditure during winter. Periodic arousals to normal Tb are energetically expensive, so hibernators trade off arousal benefits against energetic costs. This is especially important for bats with white-nose syndrome (WNS), a fungal disease causing increased arousal frequency. Little brown bats (Myotis lucifugus) with WNS show upregulation of endogenous pyrogens and sickness behaviour. Therefore, we hypothesized that WNS should cause a fever response characterized by elevated Tb. Hibernators could also accrue some benefits of arousals with minimal Tb increase, thus avoiding full arousal costs. We compared skin temperature (Tsk) of captive Myotis lucifugus inoculated with the WNS-causing fungus to Tsk of sham-inoculated controls. Infected bats re-warmed to higher Tsk during arousals which is consistent with a fever response. Torpid Tsk did not differ. During what we term “cold arousals”, bats exhibited movement following Tsk increases of only 2.2 ± 0.3 °C, compared to CloseSPigtSPi20 °C increases during normal arousals. Cold arousals occurred in both infected and control bats, suggesting they are not a pathophysiological consequence of WNS. Fever responses are energetically costly and could exacerbate energy limitation and premature fat depletion for bats with WNS. Cold arousals could represent an energy-saving mechanism for both healthy and WNS-affected bats when complete arousals are unnecessary or too costly. A few cold arousals were observed mid-hibernation, typically in response to disturbances. Cold arousals may, therefore, represent a voluntary restriction of arousal temperature instead of loss of thermoregulatory control.