Mixing and Fighting. When pigs first meet, they go through a period in which they establish social relationships. One pig dominates, and the others become subordinate. In a socially-stable pen, which is the desired environment, each pig has a stable social relationship with each other pig. Establishment of the social order requires that some, but not all, pigs fight.
Some pigs submit without a fight.
When pigs fight, they do not eat. Even the pigs who submit without fighting do not eat very much while other pigs are fighting. This reduced feed intake reduces weight gain. Mixing pigs over 150 lbs. should be avoided as much as possible. If many pigs gang up on one pig, it may be advisable to remove the one being picked on.