Thermoregulation during hibernation: application of Newton's law of cooling

Thermoregulation in the hibernating and resting little brown bat, Myotis lucifugus, was analyzed by an approach based on Newton's Law of Cooling which yielded new information on the physiology of hibernation. Standard metabolic rates were measured as carbon dioxide production rates. Experiments were grouped by experimental air temperature. The data were plotted as a function of the simultaneous body-to-ambient temperature differential. Best-fit polynomial regression lines were calculated on a computer; the mean metabolic rate curves for each ambient temperature resembled convex parabolas. Since these curves closely resembled similar plots in homeotherms the same terminology describing components of the curves was adopted: i.e. “basal metabolic rate” (BMR), “zone of thermal neutrality”, and “thermal conductance”. Correlation of studies run at the same air temperatures confirmed that the mean BMR was logarithmically related to ambient temperature with a mean Q₁₀ of 3·4 between 5° and 35°C. The theory is proposed that the logarithm of the mean thermal conductance of hibernating and resting bats is inversely related to ambient temperature. Supporting data is presented to show thermal conductance ranged from the characteristic value for homeotherms at a thermoneutral air temperature of 35°C to the calculated thermal conductance of the bat's fur alone at 5°C. It was concluded that the small hibernating bat uses thermoregulatory mechanisms similar to those of normothermic homeotherms. Newton's Law of Cooling appears to be a valid approach to analysis of heat production and heat loss mechanisms in animals at any steady-state body temperature.