Jumping gender: frogs change from she to he.

Jumping gender: Frogs change from she to he

Shrimp do it, orchids do it, even some tropical fish do it. Now biologists find that frogs do it, too--switch their sex, that is. A West German research team reports that females of two related frog species can become males without hormonal or surgical intervention. So complete is the transformation -- observed so far only in the laboratory -- that the newly male frogs breed successfully with members of their former sex.

Ulmar Grafe and Eduard Linsenmair detected the gender-bending while studying African reed frogs, Hyperolius viridiflavus ommatostictus, at the University of Wurzburg. The two were analyzing male life histories when a female began fighting with one of the males. "We were really excited, because that shouldn't happen -- females don't fight," says Grafe. In the days that followed, several females adopted the masculine mating stance, extending their forelegs and emitting a low-pitched whistle.

During the next few months, seven adult females -- including six previously observed to lay eggs -- developed functioning testicular nodules and aggressive behavior typical of male frogs, the researchers report in the current issue of COPEIA, released in January. Four of the seven "secondary males" copulated with females, fertilizing up to 70 percent of the eggs and generating normal offspring, the investigators say. Grafe and Linsenmair found that two females of a related species also changed sex in the laboratory terrariums.

"I wouldn't surprised if sex change is found in more amphibians," says biologist Robert R. Warner of the University of California, Santa Barbara, who notes that these frog species show similarities to several others.

Grafe told SCIENCE NEWS he did not see African reed frogs change gender during his recent three-month field trip to Zimbabwe, but "it's hard to imagine any animal doing this in the laboratory and not in the field. We just have to keep looking." He adds that scientists may have missed the switch in the past because the former females appear identical to ordinary males.

The sex-switching females had been housed in one of three predominantly female terrariums: a fourth terrarium with nearly equal numbers of each gender had no switches. "That behavior is very similar to [certain] fish," notes Warner. "There's a bigger reward [among females] to changing sex if there's more females around, because there's more chance to mate." A few fish species switch sex in the opposite direction, with males becoming females. Warner adds. But the key motivation, he says, appears the same: to maximize breeding.

Why, then, don't more creatures change gender? Warner speculates that the energy expenditure may be too high in species with more pronounced differences between the sexes.

It remains unclear whether reed frogs and perhaps other amphibians can change sex as easily in the wild as certain orchids, shrimp and fish do, says Grafe, who is now at Cornell University. He notes, however, that tadpoles of any species consistently develop into males after experimental exposure to the hormone testosterone. Male toads, he says, have a vestige of ovarian tissue that develops into functioning ovaries when testicular tissue is surgically removed.