Dyadic affiliative preferences in a stable group of domestic pigs

Abstract

Social preferences in pigs have received little attention despite the welfare implication of providing an adequate social environment for farm animals. We investigated the extent to which domestic pigs show affiliative preferences in three behavioural contexts—rest, exploration, social nosing—and whether these preferences were influenced by sex, relatedness and social dominance. We recorded the frequency at which pigs were associated in close spatial proximity during rest (≤30 cm) and exploration (≤1 m), and engaged in social nosing interactions (snout- snout and snout-body contacts) from age 23–29 weeks. The group consisted of 24 pigs from three mixed-sex litters of the same age, housed in an indoor pen and having free access to a large pasture area. To test for dyadic social preferences, we calculated associations in close proximity and social nosing interaction rates, classified them by strength and tested their significance with permutation methods. We then used the Multiple Regression Quadratic Assignment Procedure to test whether individual traits (sex, litter, dominance) and other social interactions (agonistic interactions, active lying down) explained the preferred partners. Preferences were overall relatively weak but found across all behavioural contexts—of all observed pairs, 44 % were preferred pairs when resting, 20 % when exploring, and 30 % when social nosing (snout-body contact). Snout-body contacts were weakly and negatively correlated with sex and dominance, whereas close proximity associations during rest and social nosing interactions were positively correlated with active lying down behaviour. Our results indicate that domestic pigs can develop preferred social relationships, and that such social preferences are weakly driven by individual attributes (sex and dominance) but influenced by the behavioural context. By shedding light on underexplored aspects of the social structure of pigs, our study strengthens the importance of accounting for the multiple drivers of social relationships to provide an adequate environment that improves management and welfare of pigs.