Reduction of metabolism during hibernation and daily torpor in mammals and birds: temperature effect or physiological inhibition?

The present study addresses the controversy of whether the reduction in energy metabolism during torpor in endotherms is strictly a physical effect of temperature (Q₁₀) or whether it involves an additional metabolic inhibition. Basal metabolic rates (BMR; measured as oxygen consumption, \(\dot V_{O_2 }\) ), metabolic rates during torpor, and the corresponding body temperatures (T _b) in 68 mammalian and avian species were assembled from the literature (n=58) or determined in the present study (n=10). The Q₁₀ for change in \(\dot V_{O_2 }\) between normothermia and torpor decreased from a mean of 4.1 to 2.8 with decreasingT _b from 30 to <10°C in hibernators (species that show prolonged torpor). In daily heterotherms (species that show shallow, daily torpor) the Q₁₀ remained at a constant value of 2.2 asT _b decreased. In hibernators with aT _b<10°C, the Q₁₀ was inversely related to body mass. The increase of mass-specific metabolic rate with decreasing body mass, observed during normothermia (BMR), was not observed during torpor in hibernators and the slope relating metabolic rate and mass was almost zero. In daily heterotherms, which had a smaller Q₁₀ than the hibernators, no inverse relationship between the Q₁₀ and body mass was observed, and consequently the metabolic rate during torpor at the sameT _b was greater than that of hibernators. These findings show that the reduction in metabolism during torpor of daily heterotherms and large hibernators can be explained largely by temperature effects, whereas a metabolic inhibition in addition to temperature effects may be used by small hibernators to reduce energy expenditure during torpor.