Winter Torpor and Roosting Ecology of Tri-Colored Bats (\u3ci\u3ePerimyotis subflavus\u3c/i\u3e) in Trees and Bridges

Subterranean hibernating tri-colored bats (Perimyotis subflavus) have experienced precipitous declines from white-nose syndrome (WNS). However, tri-colored bats also use thermally unstable roosts like tree cavities, bridges, and foliage during winter. Our objective was to determine where tri-colored bats (Perimyotis subflavus) using thermally unstable roosts lie on the torpor continuum to understand their potential WNS susceptibility, as well as determine roost use and selection in an area devoid of subterranean roosts. From November to March 2017-2019, we used temperature-sensitive radio-transmitters to track bats to their day roosts and document their torpor and activity patterns on the Savannah River Site in south-central South Carolina. We measured habitat and tree characteristics of 24 used trees and 153 random trees and used discrete choice models to determine selection. Torpid bout duration (mean 2.7 ± 2.8 days SD) was negatively related to ambient temperature and positively related to precipitation. Bats maintained a non-random arousal pattern focused near dusk and were active on 33.6% of tracked days. Of arousals, 51% contained a passive rewarming component. Normothermic bout duration, general activity, and activity away from the roost were positively related to ambient temperature, and activity away from the roost was negatively related to barometric pressure. Days were cooler (8.7°C ± 5.0) when bats used bridges than on days that they used trees (11.3°C ± 5.4). Roost selection was negatively related to stream distance and tree decay state and positively related to canopy closure and cavity abundance. Bats also appeared to favor hardwood forests and avoid pine forests. Tri-colored bats using thermally unstable roosts at SRS displayed winter torpor more reminiscent of daily torpor than classic hibernation. Our results suggest tri-colored bats in thermally unstable roosts may be less susceptible to white-nose syndrome than hibernating tri-colored bats in thermally stable roosts. Our results also suggest that access to multiple roost microclimates may be important for tri-colored bats during winter and forest management practices which retain live trees near streams with multiple roosting structures and foster cavity formation in hardwood forests will likely benefit this population. An understanding of tri-colored bat winter torpor and roosting ecology in areas devoid of subterranean roosts is increasingly important due to WNS-related declines of populations using subterranean hibernacula