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.
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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
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Snyder, G.K., Nestler, J.R. Relationships between body temperature, thermal conductance,Q 10 and energy metabolism during daily torpor and hibernation in rodents. J Comp Physiol B 159, 667–675 (1990). https://doi.org/10.1007/BF00691712
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DOI: https://doi.org/10.1007/BF00691712