T HE SEX OF AN INDIVIDUAL is a fundamental trait, determining (in animals) whether sperm or eggs are transmitted to form the next generation and thus the pattern of genetic contribution. At the same time, a panoply of behavioral, physiological, and morphological traits intrinsically linked to gonadal sex shape the specific phenotypes of individuals and hence the dynamics of populations. Furthermore, mechanisms of sex determination greatly influence the primary sex ratio and potentially the population sex ratio and effective population size, which are important ecological and evolutionary parameters. Indeed, Fisher (1930) demonstrated that under most circumstances 1:1 primary sex ratios are expected and sexdetermining mechanisms that produce such balanced sex ratios should be favored by selection (see also Bull 1983).