Social conflicts in rats induce acute autonomic nervous and neuroendocrine alterations which may be considered as a classical adaptive stress response. Besides acute and short-lasting effects, aggressive interactions sometimes result in behavioral and physiological changes that last up to weeks after the actual encounter, for instance, a suppression of explorative behavior and a reduction of the daily rhythm amplitude of activity and body temperature. Yet, these long-lasting consequences of a social conflict vary considerably between individuals.

In the present study we tried to correlate this variation to quantitative behavioral aspects of the interaction. Male wildtype rats were provided with telemetry transmitters for recording heart rate, body temperature and locomotor activity (TA11CTA-F40, Data Sciences, St. Paul, MN, USA). Heart rate and temperature were sampled for 10 sec every 10 min. Locomotor activity was recorded continuously and stored at 10-min intervals. For each individual, the daily amplitude of the rhythms of heart rate, temperature and activity were calculated as the difference between average 12h dark and 12h light values, i.e. values for circadian resting and activity phases. To assess the effects of social interactions, the daily amplitude after the tests were compared with the baseline amplitude before the interactions. The animals were confronted with a young male in their own territory for 10 consecutive days. The experimental animals systematically attacked and submitted the intruder and were clearly dominant in all the fights. These dominant-subordinate confrontations did not have any long-lasting effects on the daily rhythms of heart rate, temperature and activity. Seemingly a conflict does not have long-lasting consequences for the animal that is the aggressor and controls the fight.

Six days after the last of these dominant-subordinate interactions, the experimental animals were themselves introduced in the cage of another aggressive and pre-trained male. This confrontation between two highly aggressive animals resulted in fierce fighting and caused a strong decrease in the daily rhythm amplitude in the experimental animals. The effect lasted on average 2 weeks, however, with considerable variation between individuals. Some animals hardly showed any change while in others the rhythms only normalized after 3 weeks. The long term effects of the encounter did not seem to depend on the intensity of the conflict in terms amount of aggression received. The changes in rhythm amplitude did not correlate with the number of attacks received from the cage owner. Contrary to this, the change in amplitude showed a clear negative correlation with the aggression of the experimental animals themselves. Animals fighting back and counter attacking the cage owner subsequently had less decrease in rhythm amplitude. In line with this, there was a negative correlation between the total time spent fighting and the subsequent decrease in rhythm amplitude.

The results support the idea that the long-term consequences of a social conflict do not depend so much on the intensity of the fight but, rather, on the controllability and outcome of the encounter.