Survival of hibernating little brown bats that are unaffected by white-nose syndrome: Using thermal cameras to understand arousal behavior

White-nose syndrome is a fungal disease that has decimated hibernating bats from multiple North American species. In 2014, the invasive fungus arrived at a hibernaculum of little brown bats (Myotis lucifugus) inside the spillway of Tippy Dam, located near Wellston, Michigan, USA, yet surprisingly, this population has not experienced the declines seen elsewhere. Unlike a typical subterranean hibernaculum, light enters the spillway through small ventilation holes. We hypothesized that this light causes the hibernating bats to maintain a circadian rhythm, thereby saving energy via social thermoregulation during synchronous arousals. To test this idea, we used high-resolution thermal cameras to monitor arousals from October 2019 to April 2020. We found that arousals followed a circadian rhythm, peaking after sunset, and that most observed arousals (>68%) occurred within a cluster of bats allowing for social thermoregulation. These findings are consistent with the hypothesis that light-induced synchronized arousals contribute to the unprecedented absence of mass mortality from white-nose syndrome in this large population. Using light to maintain a circadian rhythm in bats should be tested as a potential tool for mitigating mortality from white-nose syndrome. More generally, studying populations that have been largely unaffected by white-nose syndrome may provide insight into mitigation strategies for protecting the remaining populations.