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

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Abstract
Maintenance of constant body temp. in a homoiothermic animal depends upon a balance between heat production and heat dissipation, and there are consequently 3 possible main avenues for climatic adaptation[long dash]by body-to-air gradient, by heat dissipation, and by metabolic rate. There is no evidence of adaptive low body temp. in arctic mammals and birds, or high body temp. in tropical mammals or birds. The body-to-air gradient can be adapted only by means of behavioral thermo-regulation (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 BMR is detd. by an exponential relation to size, evidently fundamental to most animals, warm blooded or coldblooded. The BMR is consequently not influenced by such factors as temp. gradient and insulation,which largely determine heat loss, and hence is inadaptive to climate. Equally inadaptive is body temp., and the phylogenetic adaptation to cold or hot climate, therefore, has taken place only through factors that regulate heat dissipation, notably the fur and skin insulation. For any temp. gradient where body temp. is maintained, the overall 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 (and critical temps.) can be approx. predicted from fur insulation and BMR. Under the same climatic conditions there may be an inverse relation between metabolic rate and fur insulation.

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