Positive welfare is more than the absence of negative experiences. Nevertheless, there are few feasible animal-based measures for positive welfare. This study aimed to investigate whether object and social play behaviours in pigs, which is believed to trigger positive emotions, is linked to peripheral oxytocin and/or serotonin changes over time. Moreover, the study examines the relationship between tail movement, play behaviour, and peripheral levels of oxytocin and serotonin in animals that have the possibility to play, along with the approach towards humans and the difficulty /ease of working with the animals. At six months old, 10 mini-pigs from the Play group participated in one or two play sessions per day for three weeks. Eight mini-pigs from a Control group did not participate in play sessions. For each first play session, blood was sampled before the play session (T-0) and 5-10 min after the play session ended (T-1). The same procedure was performed each day for three weeks. For the Control group, blood samples were drawn at the same times as the experimental group, the following day. Results showed a Play session effect on oxytocin, where a significant difference was observed for time (Chi-Square = 3.88, DF = 1, p = .05) and for group*time interaction (Chi-Square = 5.65, DF = 1, p = .02): in the Control group, T-1 was significantly higher than T-0 (p < 0.01). Regarding Play session effect on serotonin, a significant difference was observed for time (Chi-Square = 5.92, DF = 1, p = .02), between T-0 and Tj in the mean of both groups, but there was no significant difference between groups nor between group*time. No significant difference was found for the long-term effect of toy provision on oxytocin and serotonin. There was no significant difference between Play group and Control group regarding the difficulty/ease of working with and handling the pigs and their approach towards humans in our test conditions. Finally, some interesting correlations were found in the Play group during week three: positive correlations between object play frequency and tail movement duration (r = 0.84), as well as between social play duration and tail movement duration (r = 0.60); and a negative correlation between the motivation to play and the increase of peripheral serotonin between T-0 and T-1 (r = - 0.66). This study investigated the effect of toy provision in a pig experimental system on peripheral oxytocin and serotonin, as well as other parameters. The results demonstrate some interesting effects, which could be further studied for use as potential physiological and behavioural measures of positive emotions in pigs.
