Social behavioural profiles in pigs and the role of sex, dominance and kinship

Affiliative behaviours in pigs can enhance group cohesion and lower stress levels, ultimately improving individual welfare. Individual factors such as dominance rank, sex or kinship may play a key role in shaping the expression of social behaviours, but there is a lack of knowledge on the contribution of these variables. The aim of this study was to identify social behavioural profiles in pigs, based on putative affiliative behaviours, and evaluate the extent to which dominance rank, sex, and kinship influence their expression. Agonistic interactions were recorded on 212 male and female domestic pigs (Sus scrofa domesticus) to calculate dominance ranks within 16 groups. Based on this, 96 pigs (six pigs per group) were selected for observations on detailed social nosing behaviours, allogrooming, spatial proximity and social play. Principal components analysis was used to assess the presence of behavioural profiles, followed by mixed model analysis to evaluate the influence of individual factors on each principal component (PC). Snout contact constituted the majority of interactions and was exhibited by all pigs. Lying in body contact and snout-snout proximity were also frequent and were expressed by more than 95% of individuals. Allogrooming and social play were observed in 29.2 and 33.3% of pigs, respectively, and represented less than 1% of the total interaction frequency. Three PCs had eigenvalues > 1 and together explained 60.9% of the variance. The PCs related to social contact (PC1), proximity (PC2) and social engagement (PC3). Sex, dominance status and kinship had no effect on PC1 or PC3, but sex and kinship had a limited effect on PC2, with entire males showing more snout proximity than females (P < 0.001) and pigs showing less snout proximity behaviours towards their kin (P = 0.044). This study shows that the expression of putatively affiliative social behaviours can be clustered into profiles and is under commercial settings only marginally influenced by individual factors such as dominance and kinship, suggesting their general relevance to pigs' social life.