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The influence of daylength on copulatory behaviour was assessed by comparing male hamsters exposed to long or short photoperiods (14 or 2 h light/24 h). Copulation declined in animals transferred from long to short days; 13 out of 14 hamsters ceased to ejaculate within 9 weeks and many no longer intromitted in tests with sexually receptive female hamsters.
The decline in copulation in hamsters experiencing short days was associated with atrophy of the gonads and flank glands. Behavioural changes in these animals were far more gradual than those observed in hamsters after surgical castration. There was significantly more mating behaviour in tests during the subjective night of the hamsters than during their subjective day.
Exogenous testosterone was more effective in restoring copulation in castrated hamsters exposed to long days than in castrated animals experiencing short days. This suggests that in short days the substrate for copulatory behaviour is relatively refractory to androgens. Photoperiodically mediated changes in behaviour, physiology and morphology may each contribute directly to the reproductive quiescence presumed to occur in the field during the short days of autumn and winter.
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