Fritz Geiser

Hibernation versus Daily Torpor in Mammals and Birds: Physiological Variables and Classification of Torpor Patterns

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 (Tb), 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 Tbs 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 (V̇o2; 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 V̇o2 also distinguished clearly between daily heterotherms and hibernators. All other variables, including minimum Tb 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 V̇o2 during torpor.

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

SummaryThe 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,

THE TEMPORAL ORGANIZATION OF DAILY TORPOR AND HIBERNATION: CIRCADIAN AND CIRCANNUAL RHYTHMS

\dot V_{O_2 }

EVOLUTION OF DAILY TORPOR AND HIBERNATION IN BIRDS AND MAMMALS: IMPORTANCE OF BODY SIZE

), metabolic rates during torpor, and the corresponding body temperatures (Tb) 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

Seasonality of torpor and thermoregulation in three dasyurid marsupials

\dot V_{O_2 }

Torpor, thermal biology, and energetics in Australian long-eared bats (Nyctophilus)

between normothermia and torpor decreased from a mean of 4.1 to 2.8 with decreasingTb 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 asTb decreased. In hibernators with aTb<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 sameTb 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.