ALBERT

Notes: Summary 1. Male golden hamsters were exposed to either 14 h of light per 24 h (LD 14∶10) or LD 6 ∶18 for 210 days, in order to assess the effects of the light-dark cycle and changes in the reproductive system upon locomotor activity. Wheel-running activity was recorded continuously, and testicular size and serum testosterone concentrations were measured periodically throughout the study. 2. The short photoperiod induced a decrease in testicular width, serum levels of testosterone, and the number of wheel revolutions per 24-h period, when compared to the levels of these variables exhibited by LD 14∶10 hamsters. Prolonged exposure to LD 6∶18 resulted in a spontaneous increase in these three parameters to levels indistinguishable from those observed in LD 14∶10 animals. Neither testicular width, serum testosterone concentration, nor quantity of activity changed significantly in LD 14∶10 hamsters throughout the study. 3. LD 6∶18 exposure also induced an expansion of the daily activity time; this increased duration of the daily active phase persisted, even after the spontaneous increase in testicular width, serum testosterone concentration, and number of wheel revolutions per day. The duration of the daily active phase was unchanged in LD 14∶10 animals through the course of the experiment. 4. The onset of activity in all hamsters exposed to LD 14∶10 occured between 0–1 h after lights-off and did not vary appreciably for individual animals during the 210 days of LD 14∶10. The time required for stable reentrainment following a shift from LD 14∶10 to LD 6∶18 varied between 30 and 120 days among individual hamsters, and the phase relationship of activity onset to lights-off after 200 days of LD 6∶18 ranged between -1 and — 6 h. The time course of reentrainment to short days did not appear to be affected by changing testosterone levels. 5. Coincident with LD 6∶18-induced testicular regression was an increased lability of the time of activity onset each day. A return to stability in the day-to-day time of activity onset was coincident with spontaneous testicular recrudescence in LD 6∶18 hamsters. Hamsters maintained on LD 14∶10 exhibited a stable time of daily activity onset throughout the investigation. 6. These results indicate that the number of wheel revolutions per day and the lability of daily activity onset are correlated with light-induced changes in the hamster reproductive system. In contrast, the duration of the daily active phase and the phase relationship between activity onset and lights-off are relatively independent of changes in the reproductive system and are a function of the entraining light-dark cycle.