Activity and reproductive state in the hamster: independent control by social stimuli and a circadian pacemaker

Entrainment of circadian rhythms by social communication between male and female Syrian hamsters (Mesocricetus auratus) was tested by recording the wheel-running activity of pairs kept in the same cage but separated by a wire mesh barrier. Before pairing, males and females were synchronized to light/dark cycles that were 180 degrees out of phase, and at the time of pairing the hamsters were placed into constant darkness (DD). The activity rhythms of males and females housed in a cage alone (isolated) were also recorded. The freerunning periods of paired and isolated hamsters were not different over six weeks in DD, and no phase-shifts of the paired animals' rhythms were seen, indicating that the close proximity of a hamster of the opposite sex had no effect on the timing of the other's activity/rest rhythm. This was not due to a lack of communication between the paired males and females. Males showed four-day cycles in the amount and distribution of activity which corresponded to the estrous cycle of the female, and regression of the reproductive system which occurred in the isolated hamsters was delayed in both the paired males and females. Despite the fact that locomotor activity and reproduction are each regulated in part by a circadian pacemaker, social stimuli can affect both of these without influencing the circadian pacemaker that underlies the activity/rest rhythm.