Effect of social group size on aggressive behaviour between unacquainted domestic pigs

Domestic pigs (Sus scrofa) in small groups establish a dominance hierarchy using overt aggression and individual recognition. The impact of a large gr oup size on group social organisation and aggression is poorly understood. The severity of aggression on mixing unacquainted pigs derived from large g roups in which individual recognition may be impaired was studied and the i mplications for recognition of group members considered. Eight replicates o f two group size treatments (20 versus 80) were used. After 6 weeks on trea tment, a pair of pigs was introduced to a younger pair from an opponent pen in a barren arena. A total of 10 test pairs from each group were introduce d to 10 opponent pairs from a group of 20 and 10 opponent pairs from a grou p of 80, giving 320 arena tests in total. Test duration was 5 min, but pigs which initiated a fight (rapid, persistent biting) were removed immediatel y. Pigs from groups of 80 displayed a lower frequency of aggressive acts (0 .51 versus 0.90 S.E.D. 0.090 acts/pig/min) and initiated fewer fights (19.7 versus 34.6 S.E.D. 3.62% of pigs, P<0.01) in comparison with pigs from gro ups of 20. Aggression between pen mates accounted for a small amount of agg ressive acts, and was unaffected by group size. Whilst retaining the abilit y to discriminate between pen mates and foreign pigs, pigs from large group s, therefore, displayed a marked reduction in aggressive tendency towards f oreign individuals. (C) 2001 Published by Elsevier Science B.V.