The overall ability of animals to resist cold depends upon several primary factors, including insulation, feed intake, and metabolic adaptation to cold, all of which are influenced by body size. These factors affect the animal’s “critical temperature” and the temperature at which the animal exhibits maximum metabolic response to cold’ (these interrelations are shown diagramatically in Figure 1). Critical temperature is the effective ambient air temperature below which physical methods alone become insufficient in maintaining thermostability; the animal must therefore increase heat production. Below the critical temperature, an unadapted animal must either consume more feed or produce less in order to counteract the environmental thermal demand. As adaptation to cold takes place over a period of weeks, animals may increase their insulation and resting metabolic rate, thereby decreasing the critical temperature. Critical temperatures of animals under a variety of conditions of body weight, pelage, and adaptation can be calculated 3-5 on the basis of their insulation and the rate of heat production (Table 1).