Salivary Cortisol, but Not Oxytocin, Varies With Social Challenges in Domestic Pigs: Implications for Measuring Emotions

Abstract

Animals respond to inherently rewarding or punishing stimuli with changes in core affective states, which can be investigated with the aid of appropriate biomarkers. In this study we evaluate salivary cortisol (sCORT) and salivary oxytocin (sOXT) concentrations under baseline conditions and in response to two negatively- and two positively-valenced social challenges in 75 young pigs (Sus scrofa domesticus), housed and tested in eight social groups. We predicted that: (1) Relative to baseline, weaning and brief social isolation would be associated with increases in sCORT, due to psychosocial stress, and reductions in sOXT, due to a lack of opportunities for social support; and (2) Opportunities for social play, and reunions with group members after a separation would be associated with weaker sCORT responses, and increases in sOXT concentrations compared to baseline and to negative social challenges. Testing and sample collection occurred between 28 and 65 days of age and involved a within-subject design, in which every subject was sampled multiple times in neutral (baseline), negative and positive social contexts. We also recorded behavioral data and measured rates of agonism, play and affiliative interactions in the different contexts, prior to saliva sampling. As expected, negative social challenges were associated with robust cortisol responses. Relative to baseline, pigs also had higher sCORT responses to positive social challenges, although these differences were only significant during reunions. Salivary oxytocin concentrations did not differ between the different social conditions, although sOXT was lowest during the brief social isolation. Behavioral analyses confirmed predictions about the expected changes in social interactions in different social contexts, with increases in agonism following weaning, increases in coordinated locomotor play in the play context and high rates of affiliative interactions during reunions. Relative sCORT reactivity to different contexts may reflect the intensity of emotional responses, with greater increases occurring in response to challenges that involve more psychosocial stress. Our results suggest that sOXT is not a reliable indicator of emotional valence in pigs, although more research is needed to characterize sOXT responses to various challenges with and without access to social support.

Spontaneous helping in pigs is mediated by helper’s social attention and distress signals of individuals in need

“…Wild boars reside in stable, cohesive social groups (58) and forced separations from their social group can induce behavioural and physiological signs of stress in young domestic pigs (37,42). The majority of trapped pigs had increases in sCORT concentrations, and pigs with greater increases in sCORT exhibited more behavioural responses that were consistent with distress.…”

“…Pigs were tested between 46 – 55 days of age. Prior to testing, pigs were habituated to saliva sampling and had participated in one social play experiment within their social group (37) but had not been involved in other experiments. Of the 78 subjects, one died of unknown causes, and two subjects from the same group were removed from the study to be treated for illnesses before testing began.…”

“…The eluent was stored at −80°C until hormone analyses. Cortisol analyses were conducted at the FBN, using an ELISA kit for salivary cortisol (Demeditec Diagnostics®, GmbH, Kiel, Germany) that our lab has previously validated in pigs (37). The inter-assay CVs of low- and high-concentration pooled controls were 9.3% and 8.3% respectively (n = 16 samples).…”

“…We assessed the physiological responses of trapped pigs by measuring changes in the concentration of sCORT, which increases in response to physical or psychosocial challenges in mammals, including pigs (37). Saliva samples were collected by one of the authors (LRM) and two assistants using SalivaBio ® Infant Swabs (Salimetrics, CA, USA).…”

Spontaneous helping in pigs is mediated by helper's social attention and distress signals of individuals in need

“…The concordance index (CI) was 0.69, indicating good predictive discrimination of the covariates. All data and code related to this study are available online at: https://doi.org/10.5281/zenodo.8139262 [39].…”