Abstract
Small mammals in habitats with strong seasonal variation in the thermal environment often exhibit physiological and behavioral adaptations for coping with thermal extremes and reducing thermoregulatory costs. Burrows are especially important for providing thermal refuge when above-ground temperatures require high regulatory costs (e.g., water or energy) or exceed the physiological tolerances of an organism. Our objective was to explore the role of burrows as thermal refuges for a small endotherm, the pygmy rabbit (Brachylagus idahoensis), during the summer and winter by quantifying energetic costs associated with resting above and below ground. We used indirect calorimetry to determine the relationship between energy expenditure and ambient temperature over a range of temperatures that pygmy rabbits experience in their natural habitat. We also measured the temperature of above- and below-ground rest sites used by pygmy rabbits in eastern Idaho, USA, during summer and winter and estimated the seasonal thermoregulatory costs of resting in the two microsites. Although pygmy rabbits demonstrated seasonal physiological acclimatization, the burrow was an important thermal refuge, especially in winter. Thermoregulatory costs were lower inside the burrow than in above-ground rest sites for more than 50% of the winter season. In contrast, thermal heterogeneity provided by above-ground rest sites during summer reduced the role of burrows as a thermal refuge during all but the hottest periods of the afternoon. Our findings contribute to an understanding of the ecology of small mammals in seasonal environments and demonstrate the importance of burrows as thermal refuge for pygmy rabbits.
Highlights
In mid- and high-latitudes, the thermal environment can vary substantially across spatial and temporal scales, such that ambient conditions can be energetically challenging for animals
Pygmy rabbits at our study site often were exposed to thermal conditions outside of their thermoneutral zone (TNZ), but we documented seasonal differences in the relationship between temperature and energy expenditure suggesting that rabbits acclimatized to prevailing thermal conditions
Our research suggests that pygmy rabbits acclimatize seasonally by reducing energy expenditure at cold temperatures in winter relative to summer, and they have lower than expected thermal conductance
Summary
In mid- and high-latitudes, the thermal environment can vary substantially across spatial and temporal scales, such that ambient conditions can be energetically challenging for animals. Changes in space use in response to the thermal environment may allow animals to persist in habitats with unfavorable thermal conditions while minimizing energy expenditure (Huey, 1991; Williams, Tieleman & Shobrak, 1999). Burrows provide thermal refuge critical for maintaining homeothermy and reducing thermoregulatory costs during periods of extreme cold (Chappell, 1981; Harlow, 1981) and heat (Williams, Tieleman & Shobrak, 1999; Walsberg, 2000; Long, Martin & Barnes, 2005; Zungu, Brown & Downs, 2013)
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