Heat Tolerances of Australian Monotremes and Marsupials

Abstract

Heat tolerances of monotremes and marsupials have been compared by exposing animals in a hot room, on different days for 7-hr. periods, to various combinations of dry-bulb temp. ranging from 86[degree] to 108.5[degree]F with absolute humidity from 15 to 40 mm v.p. Changes in their physiological responses during heat exposure were examined. Animals studied included echidna, platypus, bandicoot, possum, cuscus, koala, wallaby, and wallaroo. Results show an evolutionary trend in homiothermism. Monotremes possess most primitive type of heat regulation, relying primarily on metabolic regulation to maintain body temperature. Sweat glands on snout and over body of platypus give it a slight advantage over echidna in hot, dry atmospheres. Marsupials show a distinct advance on monotremes in heat regulation in employing both respiratory and non-respiratory evaporative mechanisms to an increasing degree, in following ascending order bandicoot, possum, cuscus, koala, wallaby, wallaroo. Cuscus, which inhabits coastal regions of tropical Queensland and New Guinea, has a more effective evaporative mechanism than more widely distributed possum. Animals that rely to a large extent on evaporative methods for heat disposal are more distressed in humid atmospheres than those which favor other methods of heat disposal.