Female mountain goats, Oreamnos americanus, associate according to kinship and reproductive status

In fusion-fission social systems, associations are generally weak, but animals can associate preferentially, although the ultimate and proximate causes for such associations can be multiple and depend on the species. We applied a social network approach to a population of marked female mountain goats, Oreamnos americanus, characterized by a fusion-fission social system in which associations between individuals are assumed to be weak. We analysed the composition of nursery groups of mountain goats from the Caw Ridge (Alberta, Canada) population from 2005 to 2008 to test whether kinship, reproductive status and differences in body mass affect associations. We hypothesized that close kin should associate more than unrelated individuals, and we expected females of similar reproductive status, age or body mass to associate more than dissimilar individuals. We used the half-weight index correcting for differences in gregariousness (HWIG) to calculate associations between dyads of females. Females did not associate at random. Immature females associated strongly with their mothers, whereas adult females associated mainly with females of similar reproductive status. Reproductive females may associate because they share similar needs during lactation. Nonreproductive adult females and females with different reproductive status associated according to their relatedness. Body mass did not influence associations between females. To our knowledge, this is the first time that a multivariate approach has been used to describe association preferences. Our results reveal how this approach can be relevant when studying the social structure of a population because it highlights the interactions between factors influencing associations.