Aggression between unacquainted pigs—sequential assessment and effects of familiarity and weight

Abstract

Staged paired encounters between 74 pairs of 7–8 weeks old unacquainted piglets were studied. In 37 of the pairs, the pigs were pre-exposed to their opponents by keeping them for 24 h in small pens separated by wire net. The controls were kept in the same pen, separated by solid wooden walls. In 41 pairs, there was a median weight difference of 23%, and in 35 pairs the weight differed by less than 10%. The sequential assessment theory was applied to generate specific predictions about the effects of pre-exposure and of weight differences. The following results were in agreement with predictions from theory: contests were structured into a first nosing phase and a second biting phase. During the nosing phase, the rate of nose contacts was linearly dependent on the length of the phase, and during the biting phase, rate of biting was linearly dependent of length of phase. Prior to the point where one pig signalled that it gave up, there was no difference in emission rate of nosing or biting between the contestants, whereas the loser performed significantly less biting after giving up. The durations of the phases were independent of each other. The duration of the first phase was independent of weight asymmetry. Pre-exposed pairs had a reduced nosing phase, but not a reduced biting phase. However, not in agreement with predictions, weight asymmetry did not shorten contest length or biting phase and did not reduce the proportion of pairs escalating contests to overt fighting. It was also found that the variance of phase durations was smaller in pre-exposed pairs, indicating that the information obtained by the animals modified the sequence of the fights, which is not predicted by theory. In conclusion, the sequential assessment game provided explanations for a large part of the contest behaviour of pigs, but some findings were not in agreement with theory. Pre-exposure of unacquainted pigs may be a way for farmers to reduce the time of initial fighting between newly mixed pigs.