THE HEAT REGULATION OF SMALL LABORATORY ANIMALS AT VARIOUS ENVIRONMENTAL TEMPERATURES

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Abstract
Regions of thermal neutrality were found at 30[degree] to 31[degree] for the guinea pig, 28[degree] to 29[degree] for the rat, and between 30[degree] and 33[degree] for the mouse. Metabolism of the order of 590, 680, and 740 cal./sq. m./24 hrs., in the same sp. order, are typical of these temp. ranges. Above this critical area, the metabolism under hot conditions is very variable, due possibly to the lack of a true physiological regulation by evaporation. All 3 spp. make an effort at adaptation through spreading saliva on their coats, with variable success. The metabolic increment with decrease in environmental temp. is sp.-specific and highly regular, but is not in 1:1 correlation with susceptibility to heat loss as judged by surface-weight ratios, and reflects differences in peripheral insulation and probably in response adequacy. Chemical regulation in these animals is not completely effective. In spite of large increases in metabolism, the avg. body temp. falls with decreasing environmental temp. in much the same manner as has been observed in larger animals which do not show chemical regulation in this range. This fall in avg. body temp. produces failure of regulation when the physiological limit of storage is reached. Values are given for surface temps. in conjunction with known levels of rectal temp., air temp., and metabolism. From these values the various conductances, including the environmental heat loss constants, were computed. Derived values for avg. body temp. are presented as an illustration of the gradual failure of heat regulation in progressively colder environments. Theoretical considerations suggest that the practical limit of chemical regulation is not entirely dependent upon the reserve of heat producing capacity.