Physiological and behavioral effects of different success during social confrontation in pigs with prior dominance...

The impact of a 10-h social confrontation on behavior, plasma adrenaline, noradrenaline and cortisol was studied in 14 domestic pigs (eight castrated males and six females) with prior dominance experience. Prior to the experiments, animal groups, each consisting of nine growing pigs (12 weeks old) from different litters, were composed randomly. After 5 days, the pig with the highest rank from each group was removed, provided with a jugular vein catheter and kept in single housing for 2-3 weeks. After this period, each test animal was returned into its familiar group for a 10-h social confrontation. The reintroduction of the test animals into the familiar groups caused frequent agonistic interactions during the first 30 min. Seven animals succeeded to win most of their encounters during the confrontation test and were classified as High Success (HS) animals, whereas seven other animals lost most of their encounters and were classified as Low Success (LS) animals. The reintroduction of the test animals into the groups provoked also marked changes in behavioral and physiological measures. The frequent fighting behavior during the first 30 min was accompanied by a rapid increase of plasma catecholamines and a delayed increase of cortisol. Immediately after introduction, LS pigs tended to show higher plasma adrenaline and noradrenaline concentrations than HS pigs. There was also a tendency for a sustained increase of noradrenaline/adrenaline ratios in HS pigs, whereas the ratios of LS pigs remained nearly unchanged. No significant differences between both groups were found in cortisol levels and in the frequency of agonistic interactions. However, LS animals showed less locomotion, more lying and spent less time exploring the pen or other animals. These results show that during a social confrontation the experience of defeats for formerly high-ranking pigs is accompanied by increased submissive or passive behavior and a higher sympathoneural and adrenomedullary reaction, which may indicate more emotional distress and fear compared to successful animals.