Male Today, Female Tomorrow

Oct. 23, 2003 — Species that automatically change sex do so when they reach nearly three-quarters of their maximum size, neatly proving a cornerstone of evolutionary theory, scientists said.

Dozens of animal species, from types of fish and crustaceans to mollusks and worms, spontaneously change sex as a result of the pressures for survival and reproduction.

In the case of the clownfish, a favorite of aquarium-lovers, the gender bending is taken to extremes — males can not only switch to female, but also increase in size to become the alpha-breeder in their piscatorial group.

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Biologists David Allsop and Stuart West of Edinburgh University in Scotland studied 77 sex-changing species, ranging from a tiny shrimp, the Thor manningi, to a 1.5-meter (5-foot) fish called a black grouper.

They found that the creatures swapped gender when they reached 72 percent of maximum size, regardless of mating system, sex-change mechanism and other factors.

"This suggests that there is a fundamental similarity across all animals ... in the underlying forces that select for sex change," they wrote in Thursday's issue of Nature, the British weekly science journal.

Under evolutionary theory, an individual is at most pressure to change gender when there is a serious imbalance between the sexes and it has reached an age and size where it can do the switch successfully and contribute quickly to the gene pool.