Understanding and managing pig social hierarchies is essential for welfare in group housing systems, where mixing of unfamiliar animals causes aggression and stress.
Social hierarchy establishment through fighting causes significant acute welfare harm whenever pigs are mixed. Weaker or lower-ranking animals may be repeatedly attacked, causing injuries, fear, and chronic stress. In poorly managed group housing systems, subordinate sows may be unable to access feed, causing malnutrition and persistent social stress. The welfare challenge of group housing is managing the benefits of social living (meeting pigs' social needs) against the costs of social conflict (injuries and stress from hierarchy formation). Well-designed systems with adequate space, multiple resources, and stable groupings achieve the best welfare outcomes.