Effect of Falling Temperature on Heat Production in Fasting White-Tailed Deer

Heat production (energy expenditure) of fasting white-tailed deer (Odocoileus virginianus) was measured in a respiration chamber at seven successively lower levels of environmental temperature within the range of approximately 20 to -13 C. Prior to the tests, the deer were fed ad libitum on a commercially mixed pelleted ration. They were kept in an outdoor pen, and were acclimated to local weather at the time of the experiments, which were carried out from December to June. They were in thermal equilibrium with the environment at the start of the experiments, but not at subsequent lower levels of chamber temperature. Values calculated for fasting heat production (FHP) per unit of body$\text{weight}^{0.75}$at temperatures in the vicinity of 20 C fit the mouse-to-elephant curve fairly well for deer in winter coat. Corresponding values for deer in summer coat were 70 percent higher. Fasting heat production of a buck clipped in March was 91 percent higher than for full winter coat. Increases in FHP, as ambient temperature fell slightly less than 30 C over a 6-hour period, were relatively greater for deer in summer coats or clipped than for deer in winter coats. At the lowest chamber temperatures, FHP of deer in winter coats was roughly twice the interspecies mean for basal metabolic rate (BMR) of homeotherms; that of deer in summer coats was 3.5 to 5 times the expected basal rate. Seasonal differences appear to be due to the complex interactions of several factors, notably differences in insulating values of summer and winter coats, and probably seasonal differences in pituitary activity. At mean New Hampshire winter temperature, FHP per unit of$\text{weight}^{0.75}$for winter deer was almost the same as that for summer deer at mean summer temperature. It is possible that cyclical physiological changes compensate for seasonal changes in environmental temperature so that energy needs are relatively independent of temperature.