Thermogenic Capacity in Gray and Black Morphs of the Gray Squirrel, Sciurus carolinensis

During cold exposure, winter-acclimatized gray squirrels are capable of a peak metabolic rate 13.5 times their predicted standard metabolic rate (SMR). Some 20%-25% of the cold-induced heat production is due to nonshivering thermogenesis (NST). The remainder is attributed to shivering and indicates a capacity to reach a metabolic rate 10 times the predicted SMR by shivering This performance ranks among the highest reported for homeothermic animals. In contrast to shivering, NST is subjected to seasonal variation in the gray squirrel. The maximum noradrenaline-invoked thermogenesis is 3.2-3.5 times greater in cold-acclimatized than in warm-acclimatized animals. This isprimarily interpreted as a means to extend the thermal zone over which muscles can be freely allocated to activities other than shivering While the heatproduction induced by cold is independent of color morph in warm-acclimatized squirrels, black individuals have a greater NST capacity (by 11%) and a lesser rate of estimated heat loss in the cold (by 18% at -10° C) than gray ones. These results are consistent with the reported predominance of melanistic morphs in the northern part of the species' distribution.