Pigs are highly social animals that benefit from stable group living, but regrouping of unfamiliar individuals causes severe aggression events that compromise welfare — requiring careful management to balance social needs and mixing harms.
Pig social welfare presents a genuine management challenge: pigs need social companions, but mixing unfamiliar individuals triggers a welfare-costly aggression event that the industry has not eliminated. The solution is not to avoid social housing — isolated pigs show severe welfare deficits including stereotypies, reduced immunity, and impaired cognitive function — but to manage social composition to minimise the frequency and severity of mixing events. Stable groups maintained from weaning to slaughter represent the gold standard, as initial hierarchy establishment is followed by a relatively peaceful social life. The welfare problems arise in systems with repeated regrouping — at weaning, at housing transitions, and during thin/sick pig removal — each of which triggers renewed aggression. Management strategies including early mixing before weaning, providing sufficient space and escape opportunities, and using olfactory masking agents during mixing reduce but cannot eliminate welfare costs.