Pigs are highly social animals whose welfare depends on stable social groups — mixing unfamiliar pigs causes intense aggression and injuries that represent one of the most common acute welfare events in pig farming.
The biology of pig social structure makes mixing groups inherently stressful. Pigs establish dominance hierarchies through aggressive contests — mixing strangers requires re-establishing social order through fighting. The welfare harm is predictable, acute, and repeated throughout the production cycle as pigs move between housing stages. Strategies that minimize mixing — stable group housing, all-in all-out management without mixing, and gradual introductions — can significantly reduce this welfare harm, but require production system design that prioritizes social stability.