Individual coping characteristics, aggressiveness and fighting strategies in pigs

Individual pigs, Sus scrofa, differ considerably in how aggressive they are during encounters with unfamiliar conspecifics. We examined whether individual coping characteristics of pigs were predictive of aggression during social encounters and the resulting social status. Piglets were subjected to the Backtest during the suckling period, as their behavioural response in this test seems to be predictive of their coping style. Each piglet was restrained in a supine position for 1 min and the resistance (i.e. number of escape attempts) was scored. After weaning, 30 'high-resisting' (HR) and 30 'low-resisting' (LR) pigs were regrouped with unfamiliar pigs of similar weight and we recorded their aggressive behaviour for 180 min. In addition, we assessed the social rank of each pig. HR pigs showed more aggressive behaviour than LR pigs: they initiated more fights, started fighting earlier and spent more time fighting during the observation period. HR and LR pigs did not differ, however, in achieved social rank. Level of self-initiated fighting was positively correlated with social rank in LR pigs, but this relation was not found in HR pigs. In conclusion, the coping style of pigs is related to their aggressive behaviour and the establishment of dominance relationships after mixing. Our results indicate that LR pigs are flexible in using aggression, whereas the high level of aggression of HR pigs regardless of their success in encounters suggests that these animals are more rigid in their aggressive behaviour.