How do you like me now? Among bluehead wrasse, little yellow females (inset) can turn into multihued males.

Knowing When to Change Sex

Sex changes are hardly talk-show material in the sea. In fact, they're common in many marine animals, and now scientists believe they've made an important observation about how critters know the time for a change has come. Across a wide variety of species, the researchers found that sex changes occur almost exclusively in animals that have reached a particular size. The results may shed light on the evolutionary forces behind this curious phenomenon.

Animals convert from one sex to the other for a multitude of reasons. A common one is to balance out the genders in their population. For example, some female fish will transform to males if the school has too few males. The newly minted males are in great demand, and thus pass on more of their genes than if they'd stayed female. Slugs, starfish, and other creatures also switch gender when it works to their advantage. However, the cues that trigger the change vary from species to species.

To see if there might be some underlying rule, evolutionary biologists David Allsop and Stuart West of the University of Edinburgh, U.K., compiled growth rates, breeding ages, and other information on 77 marine species. The animals ranged from tiny ant-sized shrimp to fish that grow to a meter and a half long. Allsop and West's statistical analyses revealed that more than 98% of sex changers made the switch when they'd reached three-quarters of their full size, they report in the 23 October issue of Nature. "The result offers a window into the general rules that underlie the chaos we see in nature," says Allsop. He says that it suggests a fundamental similarity in the forces--whatever they may be--that shape sex-switching.

"What makes this result so interesting," says evolutionary ecologist Eric Charnov of the University of New Mexico, Albuquerque, "is that the rule generalizes over several orders of magnitude of size."

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Stuart West's lab