The effects of weight asymmetry and resource distribution on aggression ingroups of unacquainted pigs

The relationship between weight asymmetry and aggression when mixing groups of unacquainted pigs in two different environments was investigated. Ten g roups of 4 female (Landrace x Yorkshire), unacquainted pigs with a weight a symmetry of 3.1+/-0.2 kg (mean weight: 16.6 +/- 0.6 kg) between each pig, a nd another 10 groups with a weight asymmetry of 1.2 +/- 0.1 kg (mean weight : 14.1 +/- 0.1 kg), were mixed at the age of 7 weeks. Ten of the groups (fi ve of each) were mixed in an experimental pen with a heterogeneous distribu tion of straw. The pen consisted of two main compartments with straw in one of them, and a passage area with concrete floor in between. The other 10 g roups were mixed in the same pen, but with a homogenous distribution of str aw (straw spread all over the pen). The results showed that fighting duration was significantly shorter in grou ps with large weight asymmetry than in groups with small weight asymmetry i rrespective of the environment. The number of bites delivered during the fi ghts in the heterogeneous environment was lower in groups with large weight asymmetry than in groups with small weight asymmetry. In the homogenous en vironment, however, there was no significant difference between groups with large and small weight asymmetries regarding the number of bites. The comb ination of a limited straw area and a small weight asymmetry resulted in th e greatest number of bites. In groups with large weight asymmetry, the larg est pig won around 50% of the fights, and 25% of the variation in the perce ntage of fights won was explained by the weight asymmetry. In groups with small weight asymmetry, less than 10% of the variation in th e percentage of fights won could be explained by weight asymmetry. The two largest pigs delivered significantly more bites and spent significantly mor e time fighting than the smallest pig. The second largest pig received sign ificantly more bites and body lesions than the smallest pig in the groups. Fights between the two largest pigs had a significantly longer duration tha n when other pigs were involved. The results were discussed in relation to sequential assessment theory and resource defence theory. (C) 2000 Elsevier Science B.V. All rights reserved.