Patterns of natal dispersal and incest avoidance among paternally related rhesus macaques Anja Widdig,1,2,3 Andrea Trefilov,4 Fred B. Bercovitch,5 John B. Berard,6 Matthew J. Kessler,7 Jorg Schmidtke,4 Peter Nürnberg,3,8 Wolf Jürgen Streich,9 Melissa S. Gerald,2 and Michael Krawczak10 1Department of Biology, Duke University, 2Caribbean Primate Research Center, Puerto Rico, 3Institut für Medizinische Genetik, Universitätsklinikum Charité der Humboldt-Universität zu Berlin, Germany, 4Institut für Humangenetik, Medizinische Hochschule Hannover, Germany, 5Center for Reproduction of Endangered Species, Zoological Society of San Diego, 6Center for Primate Neuroethology, Neuropsychiatric Institute, UCLA, 7Center for Comparative Medicine, University of Virginia, 8Zentrum für Funktionelle Genomforschung, Universität zu Köln, Germany, 9Institut für Zoo- und Wildtierforschung, Germany, 10Institut für Medizinische Informatik und Statistik, Christian-Albrechts-Universität Kiel, Germany Abstract Most male primates leave their natal group around puberty and migrate into other social groups, probably as a means of inbreeding avoidance. Dispersal is usually associated with a high risk of injury and mortality. To maximise fitness, males are expected to co-operate with male kin during the process of dispersal and to avoid mating with close female relatives. Indeed, maternally related males are known to migrate together and support each other during dispersal, and close maternal relatives of opposite sex have been shown to avoid mating with each other. The goal of our study was to investigate the impact of paternal kinship upon male natal dispersal and inbreeding avoidance among rhesus macaques living on Cayo Santiago (Puerto Rico). Using both demographic and paternity data, we have related patterns of natal dispersal with the degree of paternal kinship. Preliminary analyses suggest that males prefer to join non-natal groups with familiar paternal half-brothers (i.e., born in the same natal group), but likewise males also prefer to join groups with familiar unrelated males. However, unfamiliar paternal half-brothers (i.e., born in different natal groups) were found to be more likely to join the same non-natal group than expected by chance alone. Furthermore, males also showed only a slight trend to avoid migration into non-natal groups containing paternal half-sister/s. Our results suggest that rhesus macaques ensure benefits in terms of fitness by migrating together with, or into the group of, a paternal half-brother and likewise avoid costs by not reproducing with paternal half-sisters.