Costs of changing sex do not explain why sequential hermaphroditism is rare.

Sex change is a relatively rare phenomenon among animals. While classic theory has been successful in assessing the adaptive significance of sex change and predicting within-species patterns, it does not explain why more animals are not sex changers. A possible explanation for the rarity of sex change is that costs such as decreased reproduction due to gonadal reconstruction favor separate sexes, or dioecy. These costs, however, have not been studied empirically or theoretically. Here, we investigate whether costs of changing sex can favor dioecy. Our analyses suggest that dioecy is favored only when costs of changing sex are large. Moreover, the fitness effect of costs and the strength of male size advantage are not static but change with the population composition, resulting in a dynamic evolutionary game between sex change and dioecy. We conclude that costs of changing sex alone are unlikely to explain the observed rarity of sex changers. Instead, assessing mating systems comparatively and quantitatively and identifying correlates selecting for and against sex change are necessary to understand why some species change sex while others do not.