Assessment of fighting ability or simple habituation: What causes young pigs (Sus scrofa) to stop fighting?

Abstract

To determine how pigs assess their relative fighting ability, I observed paired contests between young pigs that were either of similar weight or of different weight. On the first day the incidence of fighting was high, but this was not affected by the size of the weight difference. Fights between pigs of different weight were shorter than those between pigs of similar weight. When the pigs were of different weight, the eventual winner bit more frequently and spent more time attacking than did the eventual loser. This difference was not apparent when the pigs were of similar weight. On the first day, winners and losers were equally likely to initiate fights. The incidence of fighting was lower on the second day, although this was unaffected by the weight difference. Losers were less likely to initiate fights on the second day, and fights were shorter than on the first day. However, for the time that they did fight, the losers fought as vigorously (based on the rate of biting) on the second day as on the first. Seventy-two hours of visual and olfactory contact before the fight had no effect on the incidence of fighting and minimal effect on its duration. I suggest a young pig cannot assess its chance of success prior to the first encounter but only during the course of the fight. This assessment is reduced by attacks from the opponent until it drops below a critical threshold, whereupon the pig stops fighting. As a result of a previous defeat, a pig will lower its assessment and will be less likely to fight or to initiate fights on subsequent encounters. It is this change in assessment rather than habituation alone that makes pigs less likely to fight as they become acquainted.