These two sisters were separated for 5 days while the smaller one was being treated at the veterinary hospital (yes, I know that we are very likely the only people in the world who would bring a herd animal to the vet for 5 days of treatment). When pigs are introduced, or reintroduced, to one another, there can be some tension. To try to alleviate this, we placed the one returning from the vet adjacent to the pen where the other pig was for a few days so that they could become familiar with one another once more. A good plan in theory, but in the end, there was still fighting. A half-hour after this brawl, they had decided to get along (perhaps because the larger one didn't fare as well in the fight, with some damage to one of her ears), and later that afternoon, they were napping together as if nothing had happened. Still, a stressful experience for pigs and humans alike.