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Evolution of Climatic Adaptation in Homeotherms

P. F. Scholander
Evolution
Vol. 9, No. 1 (Mar., 1955), pp. 15-26
DOI: 10.2307/2405354
Stable URL: http://www.jstor.org/stable/2405354
Page Count: 12
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Evolution of Climatic Adaptation in Homeotherms
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Abstract

Mammals and birds have the same body temperature in hot and cold climates, and within the same weight range they have roughly the same basal heat production per unit weight. This leaves them with heat dissipation as the only main avenue for climatic thermal adaptation. The lowest air temperature at which a mammal or bird can rest at a basal heat production, maintaining its body temperature, is the critical temperature. This is 25-27⚬ in naked man and in many tropical species, and -40⚬ in the larger arctic species, and is a fundamental measure of the overall climatic thermal adaptation. The major reason for the low critical temperature in the arctic species is the heavy body insulation provided by fur or feather coats, combined with an outstanding tolerance of low tissue temperature (often near 0⚬) in poorly insulated peripheral parts such as legs, tail and face. There are good indications that arctic species depend upon vascular control of these poorly insulated parts for heat dissipation. Without them they would over-heat during exercise when the heat output may be increased 10-20 times over the resting rate. Aquatic species, especially, expose their naked feet, fins or tail to the most severe chilling in ice water. Yet these organs are usually thin and greatly expanded for swimming. In many arctic and tropical species cold extremities, and consequently low heat loss, are assured by arterio-venous counter current heat exchangers at the base of the limbs or tail. In spite of the low temperature which may prevail in the extremities when the animal needs heat conservation, these organs are fully functional. There is no physiological evidence, in beast or man, that the minor and erratic subspecific trends expressed in Bergmann's and Allen's rules reflect phylogenetic pathways of heat-conserving adaptation. Arctic species do not tend towards large spheres with reduced extremities and tail. The phylogenetic adaptations which have taken place are fairly well known in their main physiological aspects, as outlined above. They are major and general, as is the stimulus of the climate, and they know no taxonomic barriers.

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