Interaction between periodic socio-sexual cues and light-dark cycles in controlling the phasing of activity rhythms in golden hamsters

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Abstract
Most of the previous work on the effects of social stimuli on circadian rhythms has involved the use of constant environmental conditions. In the present experiments, male golden hamsters were maintained in a light-dark cycle and exposed to females daily for 1-hr intervals. Periodic exposure to female hamsters in estrous condition resulted in advances of the males' activity onsets on subsequent days when females were not introduced (probe days). Periodic exposure to female hamsters without regard to the stage of their estrous cycle produced similar but smaller effects. With repeated exposure to female hamsters, there were decreases in the wheel-running induced during the hour when females were present (pulse hour), perhaps reflecting habituation to socio-sexual cues. These results suggest that exposure to an arousing stimulus during the normal resting time of hamsters can result in phase shifts of their endogenous circadian activity rhythms. Non-photic events may have effects on the phasing of biological rhythms even when light-dark cues are present. Therefore, under more natural conditions, socio-sexual cues may play an important role in regulating the timing of activity in hamsters.