Unsuspected retreats: autumn transitional roosts and presumed winter hibernacula of little brown myotis in Colorado

Daniel J. Neubaum

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Documentation of autumn and winter roosts of many species of hibernating bats are lacking from western North America. However, recent evidence suggests that rather than using caves and mines, many individuals and some species of bats may roost in inconspicuous rock crevices at these times of year. I investigated autumn use of rock crevices and other roosts by the little brown myotis (Myotis lucifugus) in the Rocky Mountains of western Colorado through radiotelemetry (n = 38). Objectives were to determine the types and characteristics of roosts, describe patterns of movements to these roosts from summer colonies, and contrast findings with results of surveys of bats in cave and abandoned mines in Colorado during autumn and winter. Forty-four autumn transitional roosts and presumed hibernacula were located in buildings, trees, and rock crevices. Bats used short-distance movements changing in elevation to autumn transitional roosts and presumed hibernacula rather than major latitudinal migrations. Roost type and distance from capture site to roosts had the highest variable importance at the landscape scale. Microclimate comparisons showed that buildings provided warmer minimum average temperatures, which may benefit juvenile bats early in the transition season. Tree roost temperatures during autumn would allow bats to conserve energy by using daily torpor and passive rewarming to assist with afternoon arousals. Rock crevice roosts in talus were found to be suitable for hibernation by exhibiting the coolest average temperatures and maintaining the highest relative humidity levels. Autumn access and spring egress to high-elevation talus sites used by these bats were not obstructed by winter snow pack. These rock crevices also provided temperature and humidity levels that would support the persistence and growth of Pseudogymnoascus destructans (Pd), the causal agent of white-nose syndrome. However, bats in this study appeared to roost alone, which could inhibit the bat-to-bat spread of Pd. Surveys of caves and mines wi