Pigs are highly social animals with sophisticated dominance hierarchies and communication systems that group housing systems must accommodate for welfare.
Pigs' social complexity means that group management decisions profoundly affect welfare. Mixing of unfamiliar pigs triggers intense fighting that injures skin and ears. Early mixing during the neonatal period, before weaning, reduces later aggression. Farrowing in social groups allows piglets to develop social skills. Hiding barriers, multiple resources, and appropriate stocking density are the most evidence-based tools for reducing social stress in pigs.