Familiar versus unfamiliar: Revealing the complexity of sociability in pigs

Social behaviours are fundamental to gregarious animals and a dynamic force driving social structures, conflicts and cooperation. As a personality trait, sociability is defined as the individual reaction to the presence or absence of conspecifics and the motivation to associate with them. Research on sociability has been scarce compared to other personality traits, thus limiting our understanding of individual social behaviours which could potentially reflect it. To tackle this issue, we used a comprehensive approach, combining individual testing and Social Network Analysis. We designed a Novel Peer Test (NPT) to investigate sociability towards an unfamiliar conspecific. We conducted the NPT on 80 male domestic pigs at 6 weeks of age divided into 5 replicated groups over time. Each pig was confronted with an unfamiliar conspecific, separated by a fence in a familiar environment. To complement the NPT, we performed established personality tests, including an Open Field Test, Novel Object Test, Human Approach Test. All tests were repeated after two weeks. Pigs' social interactions in the home pen were recorded and used for Social Network Analysis, and the associated weighted degree and betweenness centrality were calculated. Through Principal Component Analysis, we found that the NPT highlighted pigs' motivation to interact with an unfamiliar conspecific, hence an aspect of sociability, and independently from novelty seeking. Additionally, the NPT may be a suitable personality test given that the behavioural profiles of the pigs were consistent over time. Pigs' network positions in the home pen for non-agonistic interactions did not predict behaviours in the NPT. Specifically, there were no links between nosing and play during the NPT and nosing and play in the home pen. This may be due to the different contexts, as the social interactions occurred dyadically with an unfamiliar conspecific during the NPT while the social networks are based on unrestrained social interactions with a group of familiar pen-mates. In contrast, walking close to the fence separating the focal and novel pig during the NPT was predicted by agonistic interactions in the home pen. This behaviour is ambiguous to interpret since it additionally loaded on the extracted sociability trait, thus, it could be driven by sociability and/or aggression. Understanding pigs' sociability will have implications for their welfare, highlighting the need for an adequate social environment to properly express different levels of sociability.