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 aggres-sion serves to establish dominance relationships, it is possible that increased initialaggression may facilitate the formation of social hierarchies. The objective of the studywas to investigate whether there is a phenotypic link between behavioural traits of aggres-sion at mixing and increased long-term group social stability. Aggressive behavioural traitswere recorded for 24 h after mixing, whereas the numbers of skin lesions (anterior, cen-tral and posterior) were obtained 24 h (SL24h) and 3 weeks post-mixing (SL3wk) for 1,166pigs. At the group level, aggressive behavioural traits were positively correlated with ante-rior 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 traitsof aggressiveness correlated positively with SL24h (0.09 to 0.53; P < 0.001), whereas theopposite associations were found for SL3wk (−0.06 to −0.14; P < 0.05). Within aggressivecohorts, animals with a high fight success rate received slightly fewer SL24h than equallyaggressive, but unsuccessful pen mates, while animals that avoided aggression received thefewest SL24h. Corresponding associations were reversed in the stable group. These resultsprovide 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 ingroups and thus benefit the long term welfare of the group.© 2015 Elsevier B.V. All rights reserved.