Dominance hierarchy in groups of pregnant sows: Characteristics and identification of related indicators

International audiencePig groups are known to be organised along dominance hierarchies. Little is known about the characteristics of the hierarchy in groups of sows and there are various methods to calculate dominance ranks within the hierarchy. Assessing dominance through recording agonistic interactions requires time-consuming observations. In order to compare the ranking methods, to investigate the sociometric characteristics of pig hierarchy such as stability, linearity and steepness, and to identify easy-to-record proxy indicators of hierarchy, four groups of 18 (± 2) gestating sows were studied. The sows were followed from 71 to 106 days of gestation. Starting from day 79, the access to the self-locking automatic feeders was limited to 6 h a day from to stimulate agonistic interactions. Each week, agonistic interactions were recorded during six continuous hours allowing the calculation of several dominance scores and sociometric variables. The restriction of feeding time increased the number of agonistic interactions but did not impact the proportion of agonistic behaviours with physical contact. The Dominance index, the David score and the Elo-score, used to rank the sow according to their dominance position, were highly inter-correlated (R > 0.92). The hierarchy was stable (stability index Stab = 0.996 ± 0.081), transitive (proportion of transitive triads Pt = 0.967 ± 0.035), quasi linear (Improved Landau index of linearity h’ = 0.701 ± 0.068) and moderately steep (real steepness Steep = 0.545 ± 0.085). The rankings obtained through Elo-score were highly correlated with the mean order feeding order (RS = 0.76), weakly correlated with the total number of feeder visits (RS = −0.21, P < 0.001) and not correlated with the daily mean time spent in the feeder (RS = 0.09, P = 0.10), or the total daily spent time in the feeder (RS = 0.10, P = 0.07). The rank was also highly negatively correlated to parity (RS = −0.83, P < 0.001) and to the weight at the end of gestation (RS = −0.75, P < 0.001). The results suggest that dominance hierarchies in stable groups of pregnant sows are stable and strong. Dominance rank of an individual can be estimated according to the feeding order, body weight or parity if there is enough variability in the group