Effects of dominance and familiarity on behaviour and plasma stress hormones in growing pigs during social confrontation.

Behavioural patterns and plasma adrenaline, noradrenaline and cortisol responses were studied in domestic pigs with different dominance status during 10-h social confrontation tests with a familiar and an unfamiliar group. Eight trials were carried out, where in each trial two groups of nine growing pigs (12 weeks old) were randomly formed. The pigs with the highest (HR) and lowest (LR) rank from each group were selected as test animals, provided with surgically implanted catheters and kept in single housing for 2 to 3 weeks. After this period, each test animal was introduced once into the familiar and once into the unfamiliar group for 10 h. Introduction of the test animals into the groups caused frequent agonistic interactions during the first 30 min and significantly more agonistic interactions during the confrontation with the unfamiliar group. The agonistic behaviour was accompanied by a rapid increase of plasma catecholamines and cortisol. HR pigs showed significantly higher plasma catecholamine concentrations and more agonistic interactions during the first 30 min compared with the LR pigs. During confrontation with the unfamiliar group, HR pigs experienced more defeats and showed a higher increase of plasma cortisol levels than during the confrontation with the familiar group. No influences of rank or familiarity were found on the other behavioural patterns examined. The results show that agonistic behaviour and activation of the sympatho-adrenomedullary and the hypothalamic-pituitary-adrenal system in pigs during a social confrontation test are determined by the former dominance rank of the animals and the familiarity of the group.