Housing immature domestic pigs in large social groups: implications for social organisation in a hierarchical society

Immature domestic pigs are increasingly housed in large social groups (>50 pigs) without an understanding of how they adapt to these conditions. At a conventional group size (typically 10-30 animals), pigs exhibit dominance relationships which are frequently established using physical assessments of strength and fighting ability and may include the use of aggressive behaviour. The relationships also rely on individual identification of group members and the recall and reinforcement of previous fight outcomes. This paper reviews the implications of a large group size for the establishment and long-term maintenance of dominance relationships in immature pigs. Evidence suggests that, in large groups, less reliance is placed on aggression during the immediate post-mixing phase than would be expected if the same proportion of dominance relationships are established using aggression in both large and small groups. This occurred despite an ability to distinguish between pen mates and non-pen mates. The energetic costs to a pig of establishing dominance relationships with a given proportion of group members is likely to increase with group size. It is considered whether these elevated costs place a ceiling on the number of fights a pig can engage in or whether they are able to assess dominance ability using passive means and thereby reduce the proportion of group members with which they fight. It is probable that a combination of these two routes is responsible for the limitation of aggression observed. Sub-grouping behaviour, investigated in large poultry groups, appears not to be adopted by pigs in response to a large group size. There is a need for further fundamental research to investigate the way in which pigs adapt behaviourally to a large group size at the level of the individual pig and several potentially fruitful investigations are suggested. (C) 2004 Elsevier B.V. All rights reserved.