Relationships between body temperature,thermal conductance,Q 10 and energy metabolism during daily torpor and hibernation in rodents

Summary In the present paper we examine the ability of rodents to maintain body temperature (T _B ) following the marked reductions in metabolic heat production associated with torpor. Previously published values for metabolic rate (M),T _B and ambient temperature (T _A ) were used to calculate thermal conductances (C') during normothermy and torpor in rodents capable of daily torpor (11 species) and hibernation (18 species). Values ofC' for torpid animals are uniformly lower thanC' in normothermic animals. In addition,C' of normothermic and torpid rodents decreases with increasing body mass (BM). However, the slope of the relationship betweenC' and BM is almost 4-fold greater for normothermic than for torpid animals. Thus, the ability of torpid rodents to conserve body heat by reducingC' decreases with increasing mass. Rodents that use daily torpor tend to be small and they tend to maintainT _B well aboveT _A during torpor. Hibernators tend to be larger and regulateT _B relatively close toT _A . Thus, the reductions inC' appear to be closely correlated with the level ofT _B regulation during torpor. We suggest that the changes inC' represent a suite of physiological adaptations that have played a central role in the evolution of torpor, enabling rodents to regulateT _B aboveT _B during periods of very low heat production. Based on the approach used here we address the controversy of whether reductions inM 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 usingQ ₁₀ to evaluate the relationship ofM andT _B in endotherms. What is perceived as metabolic inhibition (i.e.,Q ₁₀>3) is confounded by changes in the relationship ofM andT _B due to reductions inC' and reductions in the difference betweenT _B andT _A . Unfortunately, changes inM andT _B cannot be used to quantify changes in metabolic state in endotherms. Thus, neitherQ ₁₀ 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.Abbreviations T _B body temperature - T _A ambient temperature - C' thermal conductance - C _n normothermicC whenT _A is above a lower critical temperature - C _t torporC when animals are in daily torpor or hibernation - M metabolic rate - BM body mass - WVPD water vapor pressure deficit