Agonistic interactions and physiological reactions of top-and bottom-ranking pigs confronted with a familiar and an unfamiliar group: Preliminary results

Four trials were carried out, where in each trial during a selection rest two groups of growing pigs (12 weeks old) with nine animals each were randomly formed. After a week, the top (TR) and bottom (BR) ranking pig from each group was isolated and kept under single housing conditions. At the age of 15 to 16 weeks, the test animals were confronted once with the familiar and once with the unfamiliar group. During the 10-h social confrontation test, agonistic behaviour (agonistic interactions, individual dominance values) and physiological stress reactions (epinephrine, norepinephrine, cortisol, heart rate) were investigated simultaneously. The social confrontation of a pig with a familiar or an unfamiliar group caused very frequent agonistic interactions during the first 30 min. This behaviour was accompanied by a rapid increase of the heart rate and the plasma catecholamine concentrations and a delayed increase of cortisol. TR pigs elicited more agonistic interactions during the first 30 min and displayed higher plasma catecholamine concentrations. Additionally, TR animals showed significantly higher dominance values during the 10-h testing situation compared to the BR pigs. The confrontation of an animal with the unfamiliar group resulted in a more pronounced adrenocortical reaction compared to the confrontation with the familiar group. In particular, the TR animals showed a distinct increase of plasma cortisol during the first hour and a later bradycardia when they were confronted with the unfamiliar group. The preliminary results presented in this paper indicate that pigs show different, but inconsistent behavioural and physiological reactions when they are confronted with a social group. The agonistic behaviour and the physiological stress reactions during a social confrontation test are still influenced by the former rank of the animal, even when the animal was isolated under single housing conditions in the meantime for 2 to 3 weeks. (C) 1997 Elsevier Science B.V.