If sows are maintained in similar groups from gestation to gestation, aggression is reduced compared to the other regrouping strategies tested. However, this method would not always be practical. When sows were exposed to each other before breeding, aggression was similar to the control, but injuries were less. This suggests that pre-exposure may create a weak social bond, but the hierarchy still needs to be established. Providing protection during regrouping resulted in more fights, but fewer injuries, suggesting that being able to escape from fights, could prevent the establishment of a stable hierarchy. Having dominant sows in the group may result in more subtle aggression and chronic stress persisting within the group.