▪ Abstract Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (T_b) for MR reduction, perhaps with the exception of very small species and at high torpor T_b, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to T_beffects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of T_bare employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and T_breduction at torpor onset. Over the ambient temperature (T_a) range where torpid heterotherms are thermo-conforming, the T_b-T_adifferential is more or less constant despite a decline of MR with T_a; however, in thermo-regulating torpid individuals, the T_b-T_adifferential is maintained by a proportional increase of MR as during normothermia, albeit at a lower T_b. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.

Many birds and mammals drastically reduce their energy expenditure during times of cold exposure, food shortage, or drought, by temporarily abandoning euthermia, i.e. the maintenance of high body temperatures. Traditionally, two different types of heterothermy, i.e. hypometabolic states associated with low body temperature (torpor), have been distinguished: daily torpor, which lasts less than 24 h and is accompanied by continued foraging, versus hibernation, with torpor bouts lasting consecutive days to several weeks in animals that usually do not forage but rely on energy stores, either food caches or body energy reserves. This classification of torpor types has been challenged, suggesting that these phenotypes may merely represent extremes in a continuum of traits. Here, we investigate whether variables of torpor in 214 species (43 birds and 171 mammals) form a continuum or a bimodal distribution. We use Gaussian‐mixture cluster analysis as well as phylogenetically informed regressions to quantitatively assess the distinction between hibernation and daily torpor and to evaluate the impact of body mass and geographical distribution of species on torpor traits. Cluster analysis clearly confirmed the classical distinction between daily torpor and hibernation. Overall, heterothermic endotherms tend to be small; hibernators are significantly heavier than daily heterotherms and also are distributed at higher average latitudes (∼35°) than daily heterotherms (∼25°). Variables of torpor for an average 30 g heterotherm differed significantly between daily heterotherms and hibernators. Average maximum torpor bout duration was >30‐fold longer, and mean torpor bout duration >25‐fold longer in hibernators. Mean minimum body temperature differed by ∼13°C, and the mean minimum torpor metabolic rate was ∼35% of the basal metabolic rate (BMR) in daily heterotherms but only 6% of BMR in hibernators. Consequently, our analysis strongly supports the view that hibernators and daily heterotherms are functionally distinct groups that probably have been subject to disruptive selection. Arguably, the primary physiological difference between daily torpor and hibernation, which leads to a variety of derived further distinct characteristics, is the temporal control of entry into and arousal from torpor, which is governed by the circadian clock in daily heterotherms, but apparently not in hibernators.

Hibernation and daily torpor are usually considered to be two distinct patterns of heterothermia. In the present comparison we evaluated (1) whether physiological variables of torpor from 104 avian and mammalian species warrant the distinction between hibernation and daily torpor as two different states of torpor and (2), if so, whether this distinction is best based on maximum torpor bout duration, minimum body temperature ( ), minimum metabolic rate during torpor, or the reduction of metabolic rate expressed as percentage of basal metabolism (BMR). Initially, animals were grouped into species displaying either daily torpor or prolonged torpor (hibernation) according to observations from original sources. Both cluster and discriminant analyses supported this division, and further analyses were therefore based on these two groups. Frequency distributions for all tvariables tested differed significantly (P < 0.001) between daily torpor and hibernation. The average maximum torpor bout duration was 355.3 h in hibernators and 11.2 h in daily heterotherms. Mean minimum were lower in hibernators than in daily heterotherms (5.8° C vs. 17.4° C) as were minimum metabolic rates measured as rate of oxygen consumption ( ; 0.037 vs. 0.535 mL O₂ g⁻¹h⁻¹), and the metabolic rate reduction expressed as percentage of BMR (5.1% vs. 29.5%). Furthermore, mean body weights were significantly higher in hibernators (2384 g) than in daily heterotherms (253 g; P < 0.001). Thus, the comparisons of several physiological variables appear to justify a distinction between the two torpor patterns. However, of all variables tested, only the frequency distributions of maximum torpor bout duration (1.5-22 h for daily torpor; 96-1,080 h for hibernation) showed a clear gap between daily heterotherms and hibernators. The minimum also distinguished clearly between daily heterotherms and hibernators. All other variables, including minimum during torpor, did not show a complete separation between the two patterns of heterothermia. We therefore suggest that classification of torpor patterns should be based on the duration of torpor bouts or the minimum during torpor.

Mammals and birds have evolved the ability to maintain a high and constant body temperature Tb over a wide range of ambient temperatures Ta using endogenous heat production. In many, especially sma...