ADAPTATION TO COLD IN ARCTIC AND TROPICAL MAMMALS AND BIRDS IN RELATION TO BODY TEMPERATURE, INSULATION, AND BASAL METABOLIC RATE

Abstract

Maintenance of constant body temperature in a homoiothermic animal depends upon a balance between heat production and heat dissipation, and there are consequently three possible main avenues for climatic adaptation, (1) by body-to-air gradient, (2) by heat dissipation, and (3) by metabolic rate. There is no evidence of adaptive low body temperature in arctic mammals and birds, or high body temperature in tropical mammals and birds. The body-to-air gradient can be adapted only by means of behavioral thermoregulation (nest building, avoidance of direct sunshine, etc.). With few exceptions our adult arctic and tropical mammals and birds have a basal metabolic rate that fits the standard mouse to elephant curve, i.e., the basal metabolic rate is determined by an exponential relation to size; evidently fundamental to most animals, warm-blooded or not. The basal metabolic rate is consequently not influenced by such factors as temperature gradient and insulation which largely determine the heat loss, and is hence inadaptive to climate. Equally inadaptive is the body temperature, and the phylogenetic adaptation to cold or hot climate therefore has taken place only through factors that regulate the heat dissipation, notably the fur and skin insulation. For any temperature gradient where the body temperature is maintained, the over-all insulation and the metabolic rate must be so adjusted that their product is proportional to the gradient. This is confirmed by our material inasmuch as the observed critical gradients can be approximately predicted from fur insulation and basal metabolic rate. Under the same climatic conditions there may be an inverse relation between metabolic rate and fur insulation.