We investigated influences of the thermal environment on patterns of body temperature (Tb), activity, and use of burrows during the active season in a population of free-living arctic ground squirrels (Spermophilus parryii). Arctic ground squirrels normally exhibited a daily 5°C range in Tb, and had higher Tb when above ground than when in burrows (P < 0.0001). This difference decreased as standard operative temperature (Tes; an index of environmental heat load) increased. Ground squirrels entered burrows more frequently on warmer compared to average or cooler days and when Tb exceeded 39°C. On cool days with heavy precipitation, ground squirrels remained almost exclusively below ground, and peaks in Tb were associated with brief aboveground forays. Time on the surface was maximal (about 80% between 0500 and 2200 h) at Tes = 17–33°C and decreased proportionately with decreasing Tes from 17°C to −2°C. Forty-six percent of variation in timing of presence above ground could be explained by a series of thermal and nonthermal variables related to environmental heat transfer. This suggests that diurnal activity patterns in this arctic environment with 24-h daylight result from a strategy that minimizes thermoregulatory costs.
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