Keeping littermates together instead of social mixing benefits pig social behaviour and growth post-weaning

The importance of social stability and its influence on the expression of the social behaviour repertoire in domestic animals remains poorly understood, especially for affiliative behaviours and other putative socio-positive behaviours such as social play. This study investigated the occurrence and type of social behaviours, with a focus on socio-positive behaviours, and their relation to growth and welfare indicators within groups of littermates vs. groups composed of pigs from several litters and initially unfamiliar to each other. We hypothesised that pigs kept with littermates would show more positive social interactions than pigs socially mixed after weaning, and that social interactions would change over time as mixed groups progressively regained social stability. The behaviour of 14 groups of indoor-housed weaned pigs (7 groups per treatment; 8-14 pigs per group) was observed from the day after weaning (4 weeks of age) and thereafter once per week until the seventh week using scan sampling (48 scans per individual per day) and continuous observations (30 min per group per day). Pigs spent most time lying together (on average 50 % of the scans) or performing non-social behaviours (31 % of scans). Exploring together was the most common social activity, observed in 7.32 % of scans. Other social behaviours occurred infrequently and each accounted for only 0.04%-1.09% of scans. Treatment significantly influenced mounting behaviour, with more mounting between pigs in the mixed groups as compared to litter mate groups (0.52 % vs. 0.27 % of scans, respectively; P = 0.03). Other behaviours did not differ between treatments or the interaction of treatment and day. Nosing proximity, social play, exploring together, and oral manipulation of conspecifics varied between days (i.e. age; all P < 0.01). Pigs in littermate groups showed a better growth rate over the seven days after weaning (littermates: 1.03 +/- 0.11 kg vs. mixed: 0.78 +/- 0.11 kg; P = 0.01). In conclusion, socio-positive behaviours were more prevalent than socio-negative behaviours overall, supporting that positive social interactions may be an important but overlooked aspect of social life. Groups of littermates did not show more positive social behaviour than mixed groups, but they did show less mounting behaviour and seemed less affected by the stress caused by weaning.