RELATIONSHIPS BETWEEN BODY-TEMPERATURE, THERMAL CONDUCTANCE, Q10 AND ENERGY-METABOLISM DURING DAILY TORPOR AND HIBERNATION IN RODENTS

In the present paper we examine the ability of rodents to maintain body temperature (TB) following the marked reductions in metabolic heat production associated with torpor. Previously published values for metabolic rate (M), TB and ambient temperature (TA) were used to calculate thermal conductances (C') during normothermy and torpor in rodents capable of daily torpor (11 species) and hibernation (18 species). Values of C' for torpid animals are uniformly lower than C' in normothermic animals. In addition, C' of normothermic and torpid rodents decreases with increasing body mass (BM). However, the slope of the relationship between C' and BM is almost 4-fold greater for normothermic than for torpid animals. Thus, the ability of torpid rodents to conserve body heat by reducing C' decreases with increasing mass. Rodents that use daily torpor tend to be small and they tend to maintain TB well above TA during torpor. Hibernators tend to be larger and regulate TB relatively close to TA. Thus, the reductions in C' appear to be closely correlated with the level of TB regulation during torpor. We suggest that the changes in C' represent a suite of physiological adaptations that have played a central role in the evolution of torpor, enabling rodents to regulate TB above TB during periods of very low heat production. Based on the approach used here we address the controversy of whether reductions in M during torpor are due entirely to temperature effects or whether metabolic inhibition in addition to temperature effects may be important. We suggest that the controversy has been confused by using Q10 to evaluate the relationship of M and TB in endotherms. What is perceived as metabolic inhibition (i.e., Q10>3) is confounded by changes in the relationship of M and TB due to reductions in C' and reductions in the difference between TB and TA. Unfortunately, changes in M and TB cannot be used to quantify changes in metabolic state in endotherms. Thus, neither Q10 nor the approach used here can be used to make valid statements about the metabolic regulatory processes associated with torpor. Other methods, perhaps at the cell or tissue level, are needed. © 1990 Springer-Verlag.