Standard operative temperatures and cost of thermoregulation in the arctic ground squirrel, Spermophilus undulatus.

Body temperatures (T b) and daily activity patterns of free-living arctic ground squirrells (Spermophilus undulatus) were determined via telemetry at a field site in northern Alaska. Simultaneous measurements were made of ambient temperature (T a), wind speed (V), and incident solar radiation. The operative environmental temperature (T e) for ground squirrels was obtained from fur-covered, thin metal taxidermic models of the animals. Standard operative temperature (T es), a comparative index of heat flow, was calculated from T e, V, and laboratory measurements of thermal conductivity.During the period of the study (August), S. undulatus were active for about 14 h per day (06.00 to 20.00 h). T b was high throughout the daily cycle, averaging 38-39°C. Circadian variations in T b were slight; average T b values dropped <1°C at night. Daytime T b fluctuations were not closely correlated to activity or to changes in environmental conditions. Air temperatures during the study were low, usually between 10 and 15°C during the day. However, T es in exposed areas was normally higher, even though skies were generally overcast. During periods of sunshine, T es may be as high as 34°C. The absence of nocturnal activity may result from increased costs of thermoregulation at night, which sharply reduces foraging efficiency. The high and stable body temperatures of S. undulatus probably result from thermoneutral daytime T es, low activity levels, and the use of well-insulated nests.