Funder and dates: BBSRC (BB/L000393/1), 2014-2017
Aggression between pigs occurs mostly when unfamiliar pigs are mixed. Aggression is stressful for pigs, resulting in injuries, increased risk of infection, and reduced weight gain, and it represents a major animal welfare concern. The information gathering and decision making processes (i.e. assessment of whether to fight or not) used by pigs to resolve aggressive encounters are poorly understood. Game theoretical models help to distinguish between different assessment strategies that animals may apply. In the first class, termed self-assessment, animals make fight decisions based purely on their own fighting ability and stamina, using internal cues and without reference to the fighting ability of an opponent. In the second class of model, termed mutual assessment, animals self assess but also use information about the fighting ability of an opponent. Whether pigs settle fights using a self or mutual assessment strategy has major implications for the degree of escalation. It has often been assumed that pigs apply mutual assessment, but this has never been formally tested using the correct framework. This project explores assessment strategies in pigs, applying these theoretical models. In addition, the effects of personality, fighting experience, and early life circumstances are investigated. Results showed that there is large variation in aggressiveness as a personality trait in pigs. Pigs that are more aggressive attack earlier and respond more impulsively in aggressive encounters. Aggression was not a factor contributing to the fighting ability of the animal, meaning that an aggressive animal was not more likely to win than an unaggressive animal. We showed that pigs do not apply mutual assessment in contest situations, but that experience in fighting contributes to the effectiveness of the assessment abilities. Experience of fighting, as well as early life socialization of piglets, reduced aggression in future encounters. These results can contribute to reducing aggression on farm, and thereby improve animal welfare.