Herbicides may emasculate wild males. (More Frog Trouble).

New studies of male frogs in the wild link trace exposures to common weed killers with partial sex reversal. The findings suggest one possible factor behind declining amphibian populations worldwide.

Atrazine, the nation's most widely used herbicide, is nearly ubiquitous. Some U.S. rainwater carries up to 0.4 part per billion of the chemical. Earlier this year, scientists at the University of California, Berkeley reported bending the gender of male frogs by incubating tadpoles in as little as 0.1 ppb atrazine (SN: 4/20/02, p. 243). Affected animals grew extra testes and sometimes, ovaries.

Those researchers used the amphibian equivalent of a lab mouse, a frog native to South Africa. The Berkeley team now reports similar laboratory results in two U.S. species, the leopard frog (Rana pipiens) and the Pacific tree frog (Hyla regilla). It also studied wild leopard frogs from eight sites in the Midwest and West. Half came from areas of high atrazine use, but all sites had measurable concentrations in streams. Concentrations at the most heavily exposed site fluctuated over the year between 0.7 and 15.2 ppb.

At seven of those sites, 10 to 92 percent of the males had underdeveloped testes, Tyrone Hayes and his colleagues report in the Oct. 31 Nature. Furthermore, portions of some of the frogs' testes produced eggs, the researchers show in a more detailed report slated to appear in Environmental Health Perspectives. The scientists didn't detect any effects of atrazine on female frogs.

The males' malformations--where a testis appears male at the top but becomes increasingly female toward the bottom--have been witnessed only in frogs exposed to atrazine, Hayes told Science News.

"It's impossible that those gonads are functioning normally," herpetologist James Hanken of Harvard University's Museum of Comparative Zoology concluded after seeing photos from the new study.

At a symposium hosted by Tulane and Xavier Universities last month in New Orleans, Hayes described lab studies of tadpoles exposed to a blend of atrazine and metolachlor, another common herbicide. When exposed to concentrations seen in runoff from Nebraska cornfields, each, by itself, appeared to prolong metamorphosis very slightly. However, when delivered together, the pair increased the tadpoles' metamorphosis time--normally 60 days from egg to frog--by 10 days.

Hayes points out that prolonging metamorphosis increases an amphibian's exposure to atrazine while the animal is most vulnerable--during development. In fact, he's found that the most dramatic emasculating effects occur in atrazine-exposed lab frogs that develop most slowly. Hayes is now studying whether the affected frogs are even fertile.

What Hanken finds "especially worrisome" in Hayes' findings is that trace concentrations of atrazine affect frogs' testes. That's troubling, says Hanken, because "rainfall virtually everywhere now has atrazine."

Joseph Kiesecker of Pennsylvania State University in State College also worries that these herbicides "can serve as a stressor that interacts with disease outbreaks in [fostering] amphibian declines."

However, toxicologist Tim Pastoor, with the atrazine maker Syngenta in Greensboro, N.C., remains unpersuaded. He's particularly troubled by a surprising finding by the Berkeley group: that 25 ppb exposures of atrazine in the lab produced only one-third as many hermaphrodites as 0.1 ppb did. "It doesn't appeal to common sense," Pastoor says.

Hayes disagrees. Atrazine's impacts in frogs appear to stem from its stimulation of aromatase, an enzyme that converts testosterone to estrogen, he says. Normally, when gonads make too much estrogen, some of the excess enters the blood and signals the pituitary gland to shut down aromatase. However, Hayes speculates, low concentrations of atrazine probably stimulate the testes to make enough estrogen to feminize them but not enough to engage the pituitary's shutdown switch.

The big question, he and others maintain, is why atrazine appears to trigger excessive aromatase activity in the first place.