Social hierarchy formation in piglets mixed in different group compositions after weaning

The aim of this study was to investigate whether mixing of piglets in different group compositions after weaning can affect the social hierarchy formation. The linearity of the social hierarchy and the type of dyadic relationships between all possible pairs of individuals within a group were analysed using sociometric parameters. We studied the hierarchy formation in homogeneous and heterogeneous weight groups, in groups of piglets originating from two or six origin litters and in single sex groups. All these groups consisted of 12 piglets. A further treatment determined the establishment of social hierarchy in two different group sizes (six and 12 piglets per group). A total of 420 piglets in 38 groups were investigated and sociometric parameters were calculated based on the outcome of all fights observed during 72 h after mixing. Regardless of group composition, in groups of 12 piglets, quasi-linear hierarchies were established with means of improved Landau's index h′ = 0.67 and Kendall's index K = 0.62. In groups of six piglets, the social hierarchy was almost totally linear indicated by h′ = 0.9 and K = 0.88. For all group compositions there was a majority of one-way-relationships (63%, p < 0.05), i.e. dyads with wins for only one individual, followed by two-way-relationships (21%), i.e. dyads with wins for both individuals, unknown (15%) and tied relationships (3%). Groups of six piglets showed the highest percentage of one-way-relationships (74%) and the lowest percentage of tied relationships (1%) as well as of unknown relationships (2%). Thus, in smaller groups almost all dyadic relationships could be clarified. We conclude that piglets are motivated to form a linear hierarchy after mixing regardless of group composition; however, with increasing group size the degree of linearity decreases.