HOW DOES FORMER ACQUAINTANCE AFFECT AGGRESSIVE-BEHAVIOR IN REPEATEDLYMIXED MALE AND FEMALE PIGS

This study was undertaken to describe the effect of previous acquainta nce on aggressive behaviour in repeatedly regrouped pigs and the natur e of interaction between sexes. Four replicates of each 100 pigs (half entire males, half females) were weaned at 4 weeks and kept litterwis e until 8 weeks, when they were randomly allocated to 10 pens of 10 pi gs each. Random regrouping took place 4 time's at 4 week intervals. In weeks 2-4 of each mix-period, aggressive behaviour was repeatedly rec orded during feed competition. The dependent variables frequency and i ntensity of aggression were analysed (GLM-anova) in relation to class variables defined by combinations of acquaintanceship in litters and m ix-periods. Additional class variables tested were intra and intersexu al 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 th e intensity tended to be lower (P = 0.07). In mix-period 2 there was n o longer any difference between littermates, nor any difference betwee n 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 h igher intensity than those that had not been together. Intrasexual int eractions between males and females did not differ, but the frequency of male-initiated intersexual interactions was significantly higher th an female-initiated ones (P = 0.0001). No differences were found in in tensity. The frequency of aggression declined over mix-periods (P = 0. 005) which could be due to habituation or age, whereas the intensity i ncreased (P = 0.0001), mainly due to female aggression towards males. The results of this study indicate that frequent regrouping seems to b e a situation to which the social behaviour of the pig is not well ada pted, and that the mechanisms whereby pigs recognise, assess and becom e 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.