Aggression and adaptation to the light-dark cycle: role of intrinsic and extrinsic control

In wild house mice, the hypothesis that in the organization of behavior the relative contribution of intrinsic factors is more important in aggressive males, while that of extrinsic factors is more important in nonaggressive individuals was confirmed using the circadian rhythmicity of activity. The faster rate of reentrainment and the suppression of activity during a subjective night and during adaptation to the new LD cycle in the nonaggressive males indicate that their circadian rhythmicity of activity is to a large extent determined by the Zeitgeber, an extrinsic factor. The slower reentrainment rate, the lack of response to a subjective night and the normal activity level seen during reentrainment in the aggressive mice suggest strong control by the pacemaker, an intrinsic factor. Tau differences between the aggressive and nonaggressive mice provide some evidence that the pacemaker of nonaggressive males is fairly labile and is easily influenced by external factors, whereas the pacemaker of aggressive animals is rather stable.