Rhythmicity of torpor in a marsupial hibernator, the mountain pygmy-possum(Burramys parvus), under natural and laboratory conditions

Circadian rhythms have been observed in most mammals, but their importance and function remain controversial with respect to daily cycles during hiber nation. We investigated the timing of arousals from and entries into hibern ation for both free-living and captive mountain pygmy-possums (Burramys par vus). Under both natural and laboratory conditions most arousals and entrie s were entrained with the light-dark cycle. Entries occurred mainly during the night and arousals preferably around dusk, which coincides with the ons et of the normal activity phase for the nocturnal pygmy-possums. This entra inment prevailed throughout the hibernation season although only the labora tory animals were constantly subjected to photoperiodic stimuli, whereas un der natural conditions hibernacula are shielded from photic cues and diurna l temperature fluctuations. Nevertheless, possums left their hibernacula fr equently throughout winter and were occasionally trapped close to the snow surface suggesting that during the periods of post-arousal normothermia the y can be exposed to environmental stimuli. It thus appears that the synchro nisation with the photocycle was governed by a temperature-compensated circ adian clock which was reset periodically during short activity periods. For the mountain pygmy-possum, entrainment with the photocycle probably has tw o functions: 1. Entrainment ensures that foraging bouts during the hibernation season re main synchronised with the dark phase. 2. Information about the prevailing climatic conditions sampled during shor t activity periods enables them to time final spring emergence from hiberna tion when snow melt begins and ensures that the breeding season can commenc e as early as possible.