COMPETITIVE interaction between females of the same social group is characteristic of most primate species1–3. In Old World monkeys, females of high social rank maintain priority of access to scarce resources and harass low-ranking companions1–6. But different field studies have found differing correlations between female dominance and reproductive success: several populations show an advantage of rank whereas others do not1, 3, 5, 7. Although such variation may reflect divergent levels of predation, food availability or social stress in different environments, female competitive ability may also be balanced by significant reproductive costs and thus be subject to strong stabilizing selection. We report here that high-ranking female baboons (Papio cynocephalm anubis) at Gombe National Park, Tanzania, enjoy shorter interbirth intervals, improved infant survival, and accelerated maturation of their daughters. These advantages, however, are countered by a significantly higher probability of miscarriage, and a proportion of high-ranking females suffer from reduced fertility.