Heterothermy in Free‐Ranging Abert's Squirrels (Sciurus Aberti)

Body temperatures (T_b) of 7 free—ranging Abert's squirrels were monitored by intraperitoneally implanted temperature sensitive radio transmitters. Relationships between Tb, activity, solar exposure, ambient air temperature (T_a), and weather conditions were examined. T_b values ranged from 35.2 to 41.1 degrees C (° = 39.0 degrees C) and varied with behavior. A hyperthermic condition during both summer and winter developed during bouts of intense activity and the degree of hyperthermia was related to intensity of activity. Immediately following intense activity, squirrels sought shade in a tree to facilitate cooling. Squirrels exhibited hypothermia in the nest during the day and night. A diurnal rhythm of Tb was observed in free—ranging squirrels when in the nest and similar rhythms of T_b and O_2 consumption rate (V_(O_2)) were observed in laboratory squirrels. Basking was common on winter afternoons and upon emergence from the nest in the morning throughout the year, resulting in elevated Tb values. Position of the tail appeared to moderate solar irradiation striking the squirrel during basking. When positioned dorsally, the tail increased reflectance. Wind and rain significantly reduced Tb in squirrels outside the nest. The relationship between Tb and Ta was subtle and difficult to ascertain in the field. In the laboratory, Tb decreased in response to falling Ta and subsequently metabolic expenditures were depressed from the expected. The mean field Tb during summer and early fall was significantly higher than during winter or spring and may indicate seasonal acclimatization. Squirrels in the field did not cache substantial quantities of food. They foraged daily and were active during all seasons, including inclement winter weather. Fresh snow appeared to restrict movements. Squirrels were seldom outside of the nests at Ta values < —10°C. Laboratory experiments established a thermoneutral zone between 29° and 35°C. Within the zone the Vo2 was 0.69 ml O2 (g h)—1 (standard temperature and pressure, dry) or 104% of the predicted weight specific value. The curve of thermogenesis is best described by a polynomial regression due to shifts in Vo2 caused by changes in Tb in response to falling Ta. The lower lethal limit was near —20 degrees C.