Aggressive responses of isolated mice towards ‘opponents’ of differing social status

Abstract

In the present research we subjected four experimental series of mice to aggression tests. In the first series (A) we compared male mice kept in isolation with random chosen male mice raised socially. In series B we set dominant males (selected after social combat) against mice kept in isolation. In series C we set submissive individuals (defeated in social combat) against isolated mice. In the last series (D) we confronted dominant male mice with unknown submissive mice. Outcomes of the fights, analysis of encounters without fighting and analysis of body weight in series A, B and C show that isolation confers certain behavioural characteristics similar to those of social dominants as far as the aggressiveness and the tendency to attack another male are concerned. However, the outcome of fights between isolated and dominant mice (dominants obtained more victories) and the results of encounters in series C and D show clearly that isolated mice are less aggressive than dominants. Our data raise the problem as to whether the aggressive behaviour induced in isolation should be considered of pathological origin, since dominants (not socially deprived) are more aggressive towards another male even if the latter happens to be a submissive individual. One important conclusion emerges from our data: with regard to the motivation for aggressive behaviour, the “greater” aggressiveness of mice kept in isolation has been used to support Lorenz’s theory about the urge to attack. Our findings are clearly in contrast with this theory and with the postulates of the psychological-hydraulic pattern proposed by Lorenz (1950) to explain the causes of aggressive behaviour. © 1979 Taylor & Francis Group, LLC.