THE METABOLIC RATES AND BODY TEMPERATURES OF BATS

1. Oxygen consumption was measured in resting little brown bats, Myotis l. lucifugus, at ambient temperatures from 0.5° to 44.0° C. Body temperature was shown to approximate the ambient level under the conditions of the experiments, so that in effect the metabolic rate was measured over very nearly this range of body temperatures. 2. The function relating oxygen consumption to body temperature is not linear; the Q10 is 2.94 from 2° to 10° C., 5.54 from 10° to 20° C., 5.09 from 20° to 30° C., 1.69 from 30° to 37° C., and 2.22 from 37° to 41.5° C. The ratio between highest and lowest resting metabolic rates is 138:1; the temperatures represented are 41.5° and 2° C. 3. Some notes on the reaction to temperatures very close to 0° C. are included. Bats increased metabolism over that observed at 2° C. when exposed to near-freezing temperatures. The experiment was not continued long enough to find whether or not they would eventually awake. 4. The data are compared with the findings of other investigators using several species of bats. There is a close correspondence between body temperature and ambient temperature in all species, except that the tropical forms increase heat production by muscular work. Bats of the temperate zone appear to show the usual effect of size on metabolic rate per unit weight at temperatures of 30° C. or above, as Benedict (1938) has demonstrated occurs in a wide variety of homoiotherms. At 2° and at 10° C. this difference is not apparent in a comparison between these measurements and those made by Kayser (1940) on a much larger bat. 5. It is concluded that there is no physiological basis for distinguishing the daily reduction of temperature and metabolic rate in bats from that found in hibernation, except in degree. It appears that this daily phenomenon is due to a lack of thermoregulatory control. 6. Bats are considered as distinct from other heterotherms in that, at all seasons of the year, their resting temperature and metabolic rate is dependent on the ambient temperature. This also is due to their lack of thermoregulation. They are the only mammals in which the resting metabolism is a direct function of the body temperature, rather than of the ambient temperature. Nevertheless, when active, they can maintain body temperatures of 40° C. or above.