Temporal social network structures based on snout contact during group integration in pigs

During the formation of new social groups, temporal variations in behavioural associations between individuals can provide insight into the role of behaviours during group formation. While social behaviour during the establishment of new groups has been studied, there is a lack of knowledge on how non-agonistic social patterns change across time. The aim of this study was, therefore, to examine temporal variation in behavioural associations between individuals during the formation and maintenance of social relationships between conspecifics. This was studied in 15 mixed-sex groups of commercial pigs (n = 118 pigs; 8 pigs per group), which were regrouped at the start of the experiment (at weaning). They were studied between 4 and 11 weeks of age to capture variation in group stability. We focused exclusively on snout-directed behaviour given its role in conspecific recognition and affiliative interactions, whereby particularly snout-to-snout contact may contribute to the development of social relationships. Social network analysis (SNA) was used to investigate temporal associations. The results show that pigs made frequent snout contact (avg. 33 times / 15 min.) and that interactions were relatively more affiliative than aggressive (tenor range: 0.52 – 0.61), and were more affiliative in the high as compared to medium social stability phase (p = 0.02). As group integration progressed, the number of social partners involved in snout contact decreased, while the frequency of snout contact per individual increased. There was no evidence of non-random social preferences and no evidence of centralised associations (degree centralisation range: 0–0.3). Pigs showed more snout contact with non-littermates than littermates (i.e. a heterophilic association based on litter origin; assortativity range: −0.34 to −0.08) and between individuals with different early social experiences (assortativity range: −0.45 to −0.07) across all integration phases. Additionally, males initiated contact more frequently than females, whereas females occupied more central positions within the social network. These behavioural processes, which support the formation and maintenance of social relationships, show that non-agonistic behaviours such as snout contact have a prominent role in the social dynamics of pigs.