Asymmetric re‐entrainment and phase response of the circadian activity rhythm of a nocturnal marsupial(Petaurus breviceps:Phalangeridae)

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Abstract
Sugar Gliders (Petaurus breviceps) re‐entrain faster after 8‐h delay shifts of an LD 12:12 and an LD 8:16 (31–56:0.3 lux each) than after 8‐h advance shifts of these Zeitgeber cycles. In order to test whether this asymmetric re‐entrainment behavior is related to, or even caused by the phase response characteristics of the circadian system, the phase response of the activity rhythm to short and long light pulses was studied. Short light pulses (15 min of 31–56 lux against a background intensity of 0.3 lux) caused only relatively small delay shifts when applied around the onset, and more pronounced advance shifts when given at the end of the activity time (α). Onset and end of activity shifted by different amounts. Long light pulses produced by 8‐h advances and delays of one single lighttime of an LD 12:12 elicited pronounced phase delays when applied at the beginning of the activity time, but only minor phase advances when given at the posterior part of α. These results indicate that in Petaurus breviceps the phase response characteristics to long light pulses exerting parametric effects of light are responsible for the pronounced asymmetry effect in re‐entrainment. Differing phase responses of onset and end of activity point to a two‐oscillator structure of the circadian pacemaker system in this marsupial.