Natal dispersal, interactions among siblings and intrasexual competition

According to the classical model of a polygynous mating system, male-biased dispersal is a consequence of inbreeding avoidance and sexual asymmetries in competition. However, kin cooperation can change the costs and benefits of dispersal to each sex and may also select for philopatry in females. Here, we report from an experimental study designed to tease apart the effects of competition, cooperation, and inbreeding avoidance on natal dispersal in juvenile root voles (Microtus oeconomus). We manipulated the presence of opposite-sex littermates and tested how interactions among siblings influence dispersal and sexual maturation. We also manipulated the juvenile sex ratio to compare the strength of intrasexual versus intersexual competition. Natal dispersal was unrelated to the juvenile sex ratio, females aggregated in space, and there was a consistent spatial overlap among sisters. Males dispersed more in the absence of their sisters, resulting in stronger spatial segregation between sexes, than in the presence of their sisters. Thus, natal dispersal did not reduce the risks of interactions with siblings and intrasexual competition. We suggest that kin clusters in females function as a defense against aggressive or infanticidal behavior by unfamiliar males.