Early life experiences considerably influence the behavioural development of the animals in which the social environment plays a crucial role. Neonatal piglets experience intense social (including aggressive) interactions when compete with their littermates for the access to teats on the sow's udder. Competition among piglets is not of equal intensity at all parts of the sow's udder. The middle of the udder is supposed to be a much more competitive and stressful suckling environment, with a higher probability of individuals being involved in fighting with littermates. We investigated whether behavioural patterns established when suckling during lactation reappear when piglets are forced to form a new dominance hierarchy after weaning, and how piglets with different early experiences cope with a new (artificially formed) social environment. We hypothesised that aggression is much more intensive in piglets that suckle in the middle of the udder, with these individuals exhibiting more unstable patterns during the establishment of social order in the new group. Two independent experiments were completed in the present study. During the period of lactation, teat order was examined by labelling piglets according to their position of suckling (A - anterior, M - middle and P - posterior part of the sow's udder). Experiment 1 involved 120 piglets - four mixed groups were created, each containing 30 piglets from all three parts of the udder. Experiment 2 involved 80 piglets - four groups were created, each containing 20 piglets, of which one was a mixed, while the other three were separate A, M and P piglets groups. Results of the present study revealed that behavioural patterns that are established when suckling reappear when forced to form a new dominance hierarchy after weaning, and that the level of agonistic behaviour exhibited by individuals is in accordance to their suckling position. In both experiments M piglets exhibited significantly more aggression than A or P piglets (p < 0.05). The results also indicate the general instable nature of social order formation by M piglets. These findings are expected to be useful for optimising the weaning process. Therefore, the suckling position of piglets should be considered when forming weaning groups, with special attention being given to M piglets. However, retaining the natural proportion of A, M and P piglets in the weaning group remains the most effective means of reducing the level of post-weaning aggression. (C) 2014 Elsevier B.V. All rights reserved.
