Boy vs. Girl : When Balance Is Needed in Schools, Some Fish Can Change Their Sexes

While many higher animals apparently are endowed by nature with the ability to give birth to one sex over the other, some species of fish are capable of changing their sex.

This phenomenon has been examined for years by experts in the study of fish. They say that everything from predators to reproductive reasons may determine why a female fish, by way of hormone induction, may transform herself into a male.

In still other instances when sexual competition is not important, male fish may change into females, scientists say.

The ability is called hermaphroditism. It is possible because the reproductive organs of fish are not as well defined as the reproductive organs of higher species.

In some populations there may be a need for more male fish, which means that females--usually the large and strong ones--will benefit the rest of the school by becoming male.

“There are many, many families capable of doing this, especially in the tropics,” said Margaret McFall, an ichthyologist at the Scripps Institution of Oceanography in La Jolla. “Some are males first and there are others that are females first.”

Among the fish capable of changing their sex are salmon, California sheepshead and the family of fish known as labrids. The labrids include wrasses, many of which are distinguished by strong teeth, bright coloring and spiny fins, McFall explained.

In species where sexual selection is important, she said, large females will transform themselves into males to attract other females for mating.

“But even smaller males will get involved in the romance in a sort of a menage a trois ,” she said, when it comes time to fertilize the eggs. “We call these fish ‘sneaker males.’ ”

McFall explained that for many fish, size is especially important and perhaps the overriding reason why large females will at some point in their lives become males.

“If a large male is picked off by a predator, hormones will be induced in the larger female to change that female into a male. The female, by becoming a male, fills the position of a large strong male.”

For others, she said, whether to be male or female is a question of energy conservation. It’s biologically more expensive to be a female fish because the reproductive effort is greater, she explained. “When many are young and small they don’t have to put many resources into their existence, and in that sense it’s better for them to be males. Only later in life is it actually necessary to be female,” she said.