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

Abstract

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 TA 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Cooling rates and body temperature regulation of hibernating echidnas(Tachyglossus aculeatus)

“…Our results for the echidna provide no support for the suggestion (Snyder and Nestler, 1990) that entry into torpor is facilitated by changes in C.…”

“…Although our data do not include measurements of MR, we are able to make some estimates of thermal conductance (C), which in turn allows some inferences to be made about MR. It has been argued, for example, that entry into hibernation and torpor is associated with an initial increase in C, which is then reduced to levels below the euthermic minimum (Snyder and Nestler, 1990). Conductance can be calculated from thermal time constants calculated from cooling curves (Robertson and Smith, 1981), and this approach has been used extensively in studies on the thermal properties of ectothermic animals (Bakken, 1976;McNab, 2002).…”

Sex differences in the thermoregulation and evaporative water loss of a heterothermic bat,Lasiurus cinereus,during its spring migration

“…Lower conductance during torpor has also been observed in rodents (Snyder and Nestler, 1990) and other insectivorous bat species (Genoud, 1993;Hosken, 1997;Hosken andWithers, 1997, 1999;Morris et al, 1994). Conductance values for male L. cinereus during torpor were 38±5.4% of expected values based on body mass, whereas for the two females that used torpor values were 3% and 8% of those expected.…”

“…Conductance values for male L. cinereus during torpor were 38±5.4% of expected values based on body mass, whereas for the two females that used torpor values were 3% and 8% of those expected. It is unclear why conductance is sometimes lower during torpor (Snyder and Nestler, 1990), but possible explanations include changes in breathing rate, posture or circulation (Hosken and Withers, 1997).…”

Specialist-generalist model of body temperature regulation can be applied on the intraspecific level

“…However, increasing activity may lead to resource exhaustion (Russell et al, 1987), and it increases the risk of predation (Lima and Dill, 1990); nevertheless, it brings about an opportunity to find food (Overton and Williams, 2004;Sakurada et al, 2000). Torpor, in turn, is a state of regulated decrease of T b and metabolic rate (MR) (Heldmaier and Ruf, 1992;Ruf and Geiser, 2015;Snyder and Nestler, 1990), which brings about benefits when food is unavailable or when costs of foraging are too high (Hudson and Scott, 1979;Ruf and Heldmaier, 2000;Schubert et al, 2010; but see Humphries et al, 2003 andWojciechowski et al, 2011 for a discussion of increased predation risk associated with torpor). In recent decades, several studies have focused on torpor use as a response to energy deficit (Bae et al, 2003;Gutman et al, 2006;Nespolo et al, 2010;Schubert et al, 2010Schubert et al, , 2008.…”