Light-dark rhythms in aggressive behavior of the male golden hamster

A nocturnal rhythm in aggressive behavior in the male golden hamster is described. The peak frequency of aggression occurs shortly after the time of dark onset (Experiments 1 and 3). Results suggest that the rhythm is likely to be independent from similar rhythms in non-hostile social behavior and locomotor activity. The aggression rhythm is not dependent on the illumination conditions during the 10 min testing period, but is instead a function of the time of testing relative to the LD 12:12 cycle (Experiment 2). The rhythm persists for over a week in continuous illumination and requires between 2 and 4 weeks to reentrain to a 12 hr phase shift (Experiment 4). These results support the hypothesis that the aggression rhythm is endogenous to the animal. The functional significance of the aggression rhythm and its implications for future research in aggression are also discussed.