A Comparison of Thermoregulation and Evaporative Water Loss in the Hispid Cotton Rat. Sigmodon Hispidus texianus, From Northern Kansas and South‐Central Texas

Thermoregulatory abilities were examined in adult, laboratory—reared F1 offspring of wild caught adult hispid cotton rats, Sigmodon hispidus texianus, from northern Kansas and south—central Texas. These regions are representative of the northern and southern areas of the distribution of the subspecies. Responses of Kansas and Texas adults subjected to ambient temperatures Ta's) of 0—30°C are significantly different. Kansas animals have a basal metabolic rate (BMR), measured as oxygen use, of approximately 0.83 mL°g—1°h—1, which is 85% of the value predicted by mass. The BMR of Texas animals (1.03 mL°g—1°h—1) is 103% of the value predicted by mass and is significantly higher than that determined for Kansas animals. Thermoneutral zones extend between 29.5—33.7° and 33.5—35° in Kansas and Texas animals, respectively. Total and dry conductances are significantly lower in Kansas animals. Total conductances (as O2) for both Kansas (0.078 mL°g—1°h—1°°—1) and Texas (0.082 mL°g—1°h—1°°C—1) animals are higher than values predicted by body masses (113% and 115%, respectively). Body temperature is similar (38.1°) at Ta's below 35° in both Kansas and Texas animals. Evaporative water loss is similar at all Ta's, but the percentage of endogenous heat dissipated by evaporative water loss is significantly greater in Kansas animals than in Texas animals. The significantly lower metabolism and thermal conductance in the Kansas animals probably represent an adaptation for conservation of water in the more arid regions of the distribution of S. hispidus texianus. Furthermore, these adaptations are thought to have permitted S. hispidus texianus to expand its range into cooler regions of the Great Plains within the past century.