Socialising piglets before weaning improves social hierarchy formation when pigs are mixed post-weaning

Weaning of piglets involves a combination of stressors: a change of environment and diet, separation from the sow and mixing of unfamiliar litters. If litters are mixed before weaning instead, aggression and mixing stress are reduced. The present study aimed to test whether pre-weaning mixing of litters (socialisation) has longer-term benefits by reducing aggression and improving the social skills of pigs in future social encounters. Sixteen sows and their litters were housed in pens with farrowing crates. Piglets from eight litters were mixed with one other litter between 10 and 30 days of age by removing a barrier between two adjacent pens (socialised). The other eight litters were kept separately (control). On the day the litters were mixed (day 10), their sows showed signs of disturbance: they delayed their first suckling bout following barrier removal, stood more and lunged and snapped at (mainly unfamiliar) piglets, although without injuring them. Socialised piglets engaged in increased teat competition, but intervals between suckling bouts, teat fidelity and the occurrence of unsuccessful bouts were unchanged. Aggression between piglets increased as (mainly unfamiliar) piglets engaged in fighting leading to skin lesions predominantly on the front of the body. At weaning (day 30) pigs were kept in littermate groups of eight and aggressive behaviour was assessed by means of two resident-intruder tests (between days 46 and 49) and mixing pigs into groups with genuinely unfamiliar pigs from the same treatment (day 50). In resident-intruder tests, socialised pigs were more likely, and quicker, to attack a smaller unfamiliar intruder pig. After mixing, socialised pigs, when compared to controls, started fighting sooner, and these fights were shorter, although they may have been more intense, since the number of skin lesions was no different two days after mixing (day 52). Despite starting aggression more quickly, socialised pigs formed a stable hierarchy more rapidly than controls: on the day after mixing (day 51) one-sided aggression was less, fewer fights were started by subordinate pigs and by 10 days after mixing, skin lesions were fewer. In summary, mixing piglets before weaning involved some aggression between piglets, but only a temporary disruption to sow behaviour, suckling and piglet growth, as found by other authors. In addition, present findings suggest that socialised piglets learnt social skills which benefited them in the longer term, enabling them to more rapidly form stable dominance hierarchies during future aggressive encounters with unfamiliar pigs.