The Torpor-Arousal Cycle is Controlled by an Endogenous Clock

Abstract

This chapter aims at elucidating the control of the torpor-arousal cycle of mammalian hibernators, based on recent progress in circadian biology. The circadian system is now viewed as a complex network of central and peripheral clocks. In euthermy, the whole system synchronizes to a master, light-driven clock, located in the suprachiasmatic nuclei of the hypothalamus (SCN). It is proposed that in a torpor bout another, non temperature-compensated clock, the torpor-arousal clock, takes over as the master clock. According to the Arrhenius law, the endogenous period of the torpor-arousal clock will expand as temperature decreases. The corresponding subjective time will then diverge from astronomical time. Body temperature recordings of 17 complete hibernation seasons of five species have been analyzed. In astronomical time, torpor bout length (TBL) varied up to fivefold, but the corresponding durations in subjective time were remarkably constant. In all cases, the animal would simply arouse at a constant subjective time given by the torpor-arousal clock. In the frame of modern circadian biology, this suggests a much broader role of the circadian system in the control of the torpor-arousal cycle.