Resources to partition amongst reproductive activities, and theoretical models of sperm expenditure assume a basic trade-off between male investment in attracting mates and in gaining fertilizations. Recent studies of non-human animals are providing empirical evidence for this basic life-history trade-off. A number of studies have also reported short term declines in semen quality associated with social dominance. In domestic fowl, Gallus gallus domesticus, and arctic charr, Salvelinus alpinus, for example, males becoming dominant after a social challenge show a reduction in semen quality, while in cockroaches, Nauphoeta cinerea, both dominant and subordinate individuals suffer a reduction in ejaculate sperm counts resulting from the establishment of dominance hierarchies. Thus, in non-human animals, there is evidence that males trade off investment in ejaculate quality when competing for and attracting mates. In addition to being perceived as attractive, men with low pitched voices are also judged to be stronger, larger, better fighters and providers, and more dominant, and these judgments have been found to hold reasonable validity within western and hunter-gatherer societies. The negative impact on semen quality of men’s expenditure on physical training is well documented, where extreme investments in physical strength have been shown to affect the hypothalamus-pituitary-testes axis. It is thus possible that investments in traits that contribute to dominance as well as attractiveness may come at the cost of reduced semen quality. Circulating levels of testosterone are associated with decreased voice pitch, increased masculine facial features, increased dominance, and men’s success in obtaining sexual partners. Although testosterone is required within the testes to regulate spermatogenesis, high levels of circulating testosterone can impair sperm production. Indeed, testosterone supplementation has been studied as a AG-013736 VEGFR/PDGFR inhibitor potential male contraception because of its negative effects on sperm production, with increased male aggressiveness noted as a problematic side effect. Thus, elevated levels of testosterone associated with male attractiveness and dominance could suppress sperm production, mediating a negative relationship between these traits. Although significant, the effect size for the association between sperm concentration and voice attractiveness was small. Moreover, sperm concentrations were largely within the range expected for functional fertility, so that women’s preferences for men with attractive voices are unlikely to have implications for their ability to conceive. Nevertheless, even slight differences in semen quality can have considerable impact on competitive fertilization success; for example sperm velocity in fish and frogs, and sperm viability in crickets, have strong effects on fertilization success under competitive conditions, but these traits have little impact on male fertility in monogamous pairing. Thus, a weak phenotypic trade-off between attractiveness and sperm concentration is expected to have greater biological relevance in ancestral populations or natural fertility populations were females exercise polyandry. Finally we note that although in the same direction, men with low pitched voices tended to have a lower sperm concentration, this direct association was not significant.