Reproductive competition leaves traces on sex-specific genetic markers Dieter Lukas1 and Linda Vigilant1 1Department of Primatology, Max Planck Institute for Evolutionary Anthropology, Germany Abstract A central tenet of behavioral ecology is that the distribution of females is primarily the result of competition over food resources. Their distribution is, along with other factors such as predator pressure and infanticide, expected to lead to differing group structures, social relationships (despotic, nepotistic, or egalitarian) and dispersal patterns of females, which in turn are predicted to affect female reproductive success.  In order to investigate whether these different classes of female social relationships correlate with distinguishable patterns of genetic variation, we examined mitochondrial DNA (mtDNA) data by calculating the frequency distribution of pairwise sequence differences among all individuals within a social group, including related individuals. These mismatch distributions have been previously used for demographic inferences since they reflect both the distances between different sequences in a sample and the frequencies of those sequences. Based on mathematical simulations, we show that the mismatch distributions should as well reflect the breeding structure of a social species, because they are influenced by how closely or distantly related individuals are and whether some mothers have a disproportional share of offspring. For a sample of mtDNA sequence data from natural populations of 59 bird and 53 mammal species, clearly differentiated patterns independent of the demographic histories of the species were detected. These patterns correlate with descriptions of the competitive regime of females derived from behavioral or relatedness data, and when fitted to analytical expectations, in addition provide information on the degree of reproductive skew in social groups.