How does former acquaintance affect aggressive behaviour in repeatedly mixed male and female pigs?

This study was undertaken to describe the effect of previous acquaintance on aggressive behaviour in repeatedly regrouped pigs and the nature of interaction between sexes. Four replicates of each 100 pigs (half entire males, half females) were weaned at 4 weeks and kept litterwise until 8 weeks, when they were randomly allocated to 10 pens of 10 pigs each. Random regrouping took place 4 time's at 4 week intervals. In weeks 2-4 of each mix-period, aggressive behaviour was repeatedly recorded during feed competition. The dependent variables frequency and intensity of aggression were analysed (GLM-anova) in relation to class variables defined by combinations of acquaintanceship in litters and mix-periods. Additional class variables tested were intra and intersexual pairwise interactions (sex-classes). In mix-period 1 the frequency of aggression was significantly lower between littermates than between non-littermates (1.04 +/- 0.7 vs. 1.20 +/- 0.2, P = 0.03), just as the intensity tended to be lower (P = 0.07). In mix-period 2 there was no longer any difference between littermates, nor any difference between pigs that had been together in the immediately preceding mix-period and those that had not. In mix-period 4 there was a tendency for pigs that had been together also in the preceding period to interact with higher intensity than those that had not been together. Intrasexual interactions between males and females did not differ, but the frequency of male-initiated intersexual interactions was significantly higher than female-initiated ones (P = 0.0001). No differences were found in intensity. The frequency of aggression declined over mix-periods (P = 0.005) which could be due to habituation or age, whereas the intensity increased (P = 0.0001), mainly due to female aggression towards males. The results of this study indicate that frequent regrouping seems to be a situation to which the social behaviour of the pig is not well adapted, and that the mechanisms whereby pigs recognise, assess and become familiar with strangers are disturbed. The found sex differences may, however, relate to the pigs' developmental stage. (C) 1998 Elsevier Science B.V. All rights reserved.