Cold environments are challenging for small endotherms because they must increase heat production to compensate for increased heat loss to the environment. Mammals that endure cold without entering torpor may have thicker fur and a lower body temperature (Tb) than those in warm environments to save energy. Activity levels also may be reduced in the cold. We aimed to determine whether activity level, exposure to chronic cold, or an interaction between the 2 could reduce the Tb of an endotherm. Wild bush rats (Rattus fuscipes) were brought into captivity and acclimated to cold (12°C) or warm (22°C) ambient temperatures and were either sedentary or exercised. Daily exercise increased maximum Tb in cold-acclimated bush rats but also decreased minimum Tb. Mean (±SE) daily Tb was significantly lower in cold-acclimated bush rats (sedentary: 36.5°C ± 0.1°C; exercised: 36.4°C ± 0.2°C) compared to warm-acclimated bush rats (sedentary: 37.3°C ± 0.2°C; exercised: 37.0°C ± 0.1°C). Body temperature decreased significantly over the time of cold acclimation. Thicker fur and lower Tbs in cold-acclimated bush rats conferred an energy saving equivalent to 11% of daily food intake in exercised rats and 22% of daily food intake in sedentary rats. The lower Tbs recorded in some small mammals in winter are not necessarily due to lower levels of activity but can result from a regulated reduction in Tb in response to cooler ambient temperatures.
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Vol. 91 • No. 5