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

Hibernating animals use torpor [reduced body temperature (T _b) and metabolic rate] to reduce energy expenditure during winter. Periodic arousals to normal T _b 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 T _b. Hibernators could also accrue some benefits of arousals with minimal T _b increase, thus avoiding full arousal costs. We compared skin temperature (T _sk) of captive Myotis lucifugus inoculated with the WNS-causing fungus to T _sk of sham-inoculated controls. Infected bats re-warmed to higher T _sk during arousals which is consistent with a fever response. Torpid T _sk did not differ. During what we term "cold arousals", bats exhibited movement following T _sk increases of only 2.2 ± 0.3 °C, compared to >20 °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.