Is social support mediated by social relationship strength in pigs?

Social support from an individual to another can act as a buffer against stress (i.e. social buffering) for the recipient. While it is known that many species are sensitive to social buffering, including farm animals, the role of the social relationship strength remains largely unclear. The aim of this study was to investigate whether the behavioural and physiological responses of an animal exposed to a stressor were influenced by the strength of the social relationship to the partner present during the event. We hypothesised that pigs with a strong social relationship to the partner (buddy) would show a reduced stress response. Focal pigs (n = 34) of 4 months old were exposed to a mildly stressful scenario (being alone in a weighcrate), followed by the presence of a sibling with either a strong or weak prior social association. Behavioural indicators of stress and social behaviour were recorded, alongside salivary cortisol concentrations at four time points. Pigs paired with a sibling with a strong relationship had less often their snouts in proximity, and for a shorter total duration, and had less often their heads in proximity. They also made more low-pitch vocalisations. Cortisol concentrations increased following the event, but no significant differences were observed between treatment groups. Buddies mostly remained calm but showed variation in their contact-seeking behaviour. Female buddies differed from males in their ability to provide social support. These findings suggest that the relationship strength has an influence on the stress-related behaviour of the focal pig but may not be sufficient to elicit measurable hormonal changes. Overall, the results indicate that social relationships between pigs are complex, with close companionship being important for their ability to cope with stressful events.