Although the sex ratios of many groups conform to Fisher's (1930) prediction that parents should invest equally in daughters and sons, a number of taxa are characterized by excesses of one gender. A variety of mechanisms may lead to sex ratio biases, but in organisms that reproduce clonally as well as sexually, gender differences in the rate of cloning could drive the development of sex-ratio biases. In this study, I demonstrate that males of the clonal brittle star Ophiactis savignyi were significantly more likely to divide than females and that the magnitude of this difference was sufficient to explain the consistent and significant excess of males in natural populations. Females were significantly more likely to lose sexual reproductive capabilities following division, and this greater cost associated with division may explain why females are less likely to divide. Gender differences in mortality rates are unlikely to explain the excess of males in this species. Because of their potential influence on the operational sex ratio, gender differences in division rates may have important ecological and evolutionary implications including effects on the direction and strength of selection.

Corresponding Editor: R. Burton