Pitch is the most perceptually salient feature of the voice, yet it is approximately five standard deviations lower in men than in women, a degree of sexual dimorphism exceeding that of all extant nonhuman apes. Evidence from Western samples suggests that low-frequency vocalizations may have augmented male mating success ancestrally by intimidating competitors and/or attracting mates. However, data are lacking from small-scale societies. We therefore investigated sexual selection on male pitch (measured by fundamental frequency, f o ) in a population of Bolivian forager-horticulturists, the Tsimané. We found that experimentally lowering f o in audio clips of men speaking increased perceptions of fighting ability but did not affect perceptions of prestige and decreased their attractiveness to women. Further, men with lower speaking f o reported higher numbers of offspring, and this was mediated by the reproductive rates of men's wives, suggesting that men with lower f o achieved higher reproductive success by having access to more fertile mates. These results thus provide new evidence that men's f o has been shaped by intrasexual competition.