Torpor in mammals and birds

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Abstract
Torpor in mammals and birds is characterized by a periodic lowering of the set point for body temperature regulation to achieve a hypometabolic state for energy and water conservation. Torpor may be seasonal (hibernation, estivation) or non-seasonal (nocturnal hypothermia, daily torpor) and, depending on the depth and duration of the torpor, energy savings from 10 to 88% may be achieved. Despite the different patterns of torpor, recent studies have demonstrated a physiological homology between sleep and torpor, suggesting a continuum in energy conservation in times of inactivity. The physiological and biochemical adaptations for torpor are many, including both species-specific and seasonally dependent aspects. Two examples are discussed. In terms of cardiac function, differences exist among species in the maintenance of resting and action potentials at low temperatures, whereas seasonal differences exist for dependence of trans-sarcolemmal calcium flux in excitation–contraction coupling. With respect to ionic regulation, the ability of red blood cells to maintain high intracellular K+ and low intracellular Na+ and Ca2+ at low temperatures is species-specific and not seasonally dependent. The competence for intracellular Ca2+ regulation (i.e., active calcium pump) at low temperatures appears to be a key feature that allows the hibernator's cells to function continually despite the prolonged and profound depression of body temperature during hibernation.

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