Analysis of the phenotypic link between behavioural traits at mixing and increased long-term social stability in group-housed pigs

Mixing of growing pigs results in aggressive contests between group members. As aggression serves to establish dominance relationships, it is possible that increased initial aggression may facilitate the formation of social hierarchies. The objective of the study was to investigate whether there is a phenotypic link between behavioural traits of aggression at mixing and increased long-term group social stability. Aggressive behavioural traits were recorded for 24 h after mixing, whereas the numbers of skin lesions (anterior, central and posterior) were obtained 24 h (SL24h) and 3 weeks post-mixing (SL3wk) for 1,166 pigs. At the group level, aggressive behavioural traits were positively correlated with anterior SL24h (0.34 to 0.67; P < 0.01) at mixing, and negatively with central SL3wk (−0.28 to −0.38; P < 0.01) in the stable group. At the individual animal level, most behavioural traits of aggressiveness correlated positively with SL24h (0.09 to 0.53; P < 0.001), whereas the opposite associations were found for SL3wk (−0.06 to −0.14; P < 0.05). Within aggressive cohorts, animals with a high fight success rate received slightly fewer SL24h than equally aggressive, but unsuccessful pen mates, while animals that avoided aggression received the fewest SL24h. Corresponding associations were reversed in the stable group. These results provide evidence that increased aggression at mixing may aid stable hierarchy formation. This raises an ethical dilemma in pigs production, but potentially also in other species, that increased acute aggression during mixing may actually decrease chronic aggression in groups and thus benefit the long term welfare of the group.