The temporal organization of daily torpor and hibernation: Circadian and circannual rhythms

Mammals and birds have evolved the ability to maintain a high and constant body temperature Tb over a wide range of ambient temperatures T-a using end ogenous heat production. In many, especially small endotherms, cost for the rmoregulatory heat production can exceed available energy; to overcome thes e energetic bottlenecks, they enter a state of torpor (a regulated reductio n of Tb and metabolic rate). Since the occurrence of torpor in many species is a seasonal event and occurs at certain times of the day, we review whet her circadian and circannual rhythms, important in the timing of biological events in active animals, also play an important role during torpor when T b is reduced substantially and may even fall below 0 degrees C. The two dis tinct patterns of torpor, hibernation (prolonged torpor) and daily torpor, differ substantially in their interaction with the circadian system. Daily torpor appears to be integrated into the normal circadian rhythm of activit y and rest, although torpor is not restricted only to the normal rest phase of an animal. In contrast, hibernation can last for several days or even w eeks, although torpor never spans the entire hibernation season, but is int errupted by periodic arousals and brief normothermic periods. Clearly, a da y is no longer divided in activity and rest, and at first glance the role o f the circadian system appears negligible. However, in several hibernators, arousals not only follow a regular pattern consistent with a circadian rhy thm, but also are entrainable by external stimuli such as photoperiod and T -a. The extent of the interaction between the circadian and circannual syst em and hibernation varies among species. Biological rhythms of hibernators for which food availability appears to be predictable seasonally and that h ibernate in deep and sealed burrows show little sensitivity to external sti muli during hibernation and hence little entrain-ability of arousal events. In contrast, opportunistic hibernators, which sometimes use arousals for f oraging and hibernate in open and accessible hibernacula, are susceptible t o external zeitgebers. In opportunistic hibernators, the circadian system p lays a major role in maintaining synchrony between the normal day-night cyc le and occasional foraging. Although the daily routine of activity and rest is abandoned during hibernation, the circadian system appears to remain fu nctional, and there is little evidence it is significantly affected by low T-b.