Torpor minimizes energy expenditure and water loss during periods of inclement weather, low prey availability, or both, and appears especially important in harsh environments such as deserts. We investigated use of torpor by free-ranging adult male pallid bats (Antrozous pallidus) at the northern limit of their range in the semiarid Okanagan Valley in British Columbia, Canada. We predicted that males would use torpor frequently during the active season and that ambient temperature (Ta) as well as nutritional condition (body condition index [BCI]) would affect torpor use. We expected roost microclimate to be conducive to daily heterothermy given that roosts had cool morning temperatures that increased in the afternoon. We used temperature-sensitive radiotransmitters to measure skin temperature (Tskin) of 8 bats from June to August 2003. Seven individuals used torpor each day (n = 54 tracking days) but not at night. Torpor duration increased as mean daytime Ta decreased albeit the relationship differed between individuals. There was no significant effect of individual on the relationship between minimum Tskin during torpor and Ta. BCI was significantly and inversely correlated with torpor bout duration. Mean daytime temperatures in rock crevice roosts approached 30°C and fluctuated less than Ta.
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