Individual aggressiveness measured in a resident-intruder test predicts the persistence of aggressive behaviour and weight gain of young pigs after mixing

Abstract

Young growing pigs of both sexes housed in littermate groups of eight were given an aggressiveness score based on two resident-intruder tests. They were then mixed into new groups of eight, composed of four randomly selected pigs from each of two litters, and behaviour was observed for 2 days (days 1 and 2). Aggressive social behaviour at mixing was analysed using weight and aggressiveness score (at both the group and individual levels) as explanatory variates. On day 1: heavier pigs in each group were more involved in fighting, won more fights, and also initiated more brief aggressive acts (headknocks and bites). Bullying was more frequent in groups with higher aggressiveness scores, and aggressive pigs also initiated more headknocks. On day 2: pigs with high aggressiveness scores persisted in showing aggressive behaviour, initiating fights, headknocks and chases. Aggressiveness score was now also associated with fighting success (in addition to weight). On both days, pigs preferred to lie with littermates rather than unfamiliar pigs, although this preference was less marked on day 2. A stronger preference for littermates was shown in heavier groups. A greater number of skin lesions were recorded at the beginning of day 2 in groups containing heavier pigs and/or pigs with higher aggressiveness scores. Weight gain in the 5 days following mixing was lower in groups containing pigs with high aggressiveness scores. Overall, weight and aggressiveness score (measured in a resident-intruder test) each affected different aspects of aggressive behaviour at mixing. While weight predicted success in agonistic interactions, aggressiveness affected the nature and time scale of interaction: high aggressiveness score pigs showed more one-sided aggression (bullying) and their aggressive behaviour was more persistent, continuing into the day after mixing. Thus, aggression at mixing can be reduced by mixing smaller pigs, and identifying a means of reducing individual aggressiveness would be worthwhile.