The chronobiology of the brushtail possum, Trichosurus vulpecula (Marsupialia: Phalangeridae):: tests of internal and external control of timing

The chronobiology of the brushtail possum (Trichosurus vulpecula) was investigated in a vivarium and in light-controlled cabinets to determine what controls the timing of natural patterns of activity and rest. It is proposed that the timing of natural behaviour of the possum is not regulated entirely by direct response to environmental factors, but that it may have an element of internal control. Unless perturbed by wind and/or rain, the onset of activity is precisely timed each day, beginning as light intensity declines following sunset. In tests of an internal clock hypothesis, possums in constant darkness exhibited free-running circadian rhythms of activity with periods initially slightly shorter than 24 h, spontaneously reducing to c. 22 h 40 min after c. 40 days. The internal rhythm of the possum could be entrained by 24-h light/dark cycles with activity initiated at the onset of the dark phase. We propose that the timing of the onset of natural behaviour of the possum is controlled through the output of a circadian clock that may be modulated by direct responses to wind and rain.