Sex change and the size-advantage model.

At the intraspecific level, the size-advantage model attempts to explain why some species change sex and others do not. Generally, sex change is favored if the relation between reproductive success and size or age differ between the sexes. Differential costs of reproduction and the mating system appear to be among the major factors producing differences in reproductive expectations between the sexes. On the intraspecific level, tests of the model consist of predicting the optimal size or age of sex change. In general, an individual should change sex if it can increase its reproductive value (future expected reproduction) by doing so. Reproductive values are often dependent on the local environment, local population demography, and the individual's own status, and labile sex change appears to be common. Many of the objections to the size-advantage model arise from confusion between the intraspecific and intraspecific versions.