Body temperature and activity patterns in free-living arctic ground squirrels

We investigated influences of the thermal environment on patterns of body temperature (T-b), 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 degrees 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 (T-es; 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 degrees 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 T-es = 17-33 degrees C and decreased proportionately with decreasing T-es from 17 degrees C to -2 degrees 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.