Winter activity of tricolored bats in aboveground and subterranean hibernacula in the southeastern USA

Susceptibility of bats to white-nose syndrome (WNS), a lethal disease caused by the fungus Pseudogymnoascus destructans (Pd), may be influenced by the amount of activity outside hibernacula during the winter. We tested the effects of hibernaculum type (aboveground or subterranean) and Pd status (positive or negative) on winter activity of tricolored bats (Perimyotis subflavus) in the southeastern USA along with the effects of ambient temperature, precipitation, and stage of hibernation. We placed acoustic detectors at the entrances of 13 hibernacula (4 aboveground and Pd-positive, 4 aboveground and Pd-negative, 4 subterranean and Pd-positive, and 1 subterranean and Pd-negative) during winter 2020-21 and 2021-22. While neither hibernaculum type nor Pd status alone predicted probability of activity or levels of activity, these factors interacted with temperature, precipitation, and stage of the hibernation period. Activity increased at a greater rate with temperature and time since the onset of hibernation in aboveground and Pd-negative sites and decreased at a faster rate in response to precipitation. Our results suggest that tricolored bats using aboveground hibernacula such as culverts or bridges may be less susceptible to WNS due to greater nighttime activity. However, use of these structures may have other costs such as higher freezing and predation risks.