Researcher: Pesticide 'Castrates' Male Frogs Atrazine is widely used as weedkiller on American farms. And a new study shows this common chemical may have gender-bending effects on frogs. Host Guy Raz talks to biology professor Tyrone Hayes about his work with atrazine and frogs. Hayes found that 9 of every 10 male frogs he exposed to atrazine became chemically castrated. And that other 1 out of every 10? Well, he became a she.
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Researcher: Pesticide 'Castrates' Male Frogs

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Researcher: Pesticide 'Castrates' Male Frogs

Researcher: Pesticide 'Castrates' Male Frogs

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  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
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GUY RAZ, host:

The film was called "Junior," and her leading man was Arnold Schwarzenegger. And (unintelligible) that movie could be summarized in one short sentence.

(Soundbite of film, "Junior")

Governor ARNOLD SCHWARZENEGGER (Republican, California): (As Dr. Alex Hesse) I'm pregnant.

Ms. EMMA THOMPSON (Actor): (As Dr. Diana Reddin) Good explanation.

Gov. SCHWARZENEGGER: (As Hesse) I really am pregnant.

RAZ: The story might be Hollywood fantasy to humans, but to frogs, well, that's a different matter. A group of biologists has just published a study that says the chemical Atrazine can actually turn a male frog into an egg-laying female.

Atrazine is one of the most widely used herbicides in the United States. It's been banned in Europe because of its suspected links to other abnormalities in amphibians. Here in the U.S., though, the EPA gave Atrazine the all-clear in 2006. But now, the agency has decided to take another look.

Tyrone Hayes led that study on Atrazine and frogs at U.C. Berkeley, and he joins me from there. Welcome.

Professor. TYRONE HAYES (Biology, University of California Berkeley): Thank you. Good to be here.

RAZ: How does Atrazine work? I mean, how does this chemical turn a male frog into a female?

Prof. HAYES: Well, in part, we're still working on that, but what we believe now is that Atrazine turns on an enzyme called aromatase, and this enzyme is the machinery, if you will, that converts testosterone into estrogen.

So we exposed frogs starting from the tadpole stage all the way out to three years, and what we found was that first of all, about 10 percent of the animals turned completely into females that were capable of laying eggs or reproducing with unexposed males, and the males that didn't turn into females, those were chemically castrated.

So they had low testosterone, low sperm. They didn't mate with females, chemically castrated.

RAZ: Now, there have been attempts to link Atrazine, this herbicide, to increased rates of breast cancer in people, none of which have been proven, I understand. Is there any sense that Atrazine does affect humans?

Prof. HAYES: Well, there's at least one study that was conducted in Kentucky, and that was a fairly large study. And that study concluded that women whose well water was contaminated with Atrazine were more likely to develop breast cancer, and the link appeared to be to Atrazine.

RAZ: We called up Tim Pastoor. He's the chief scientist at Syngenta.

Prof. HAYES: Yes, I know him well.

RAZ: Which is the company that makes Atrazine. He called your study seriously flawed. He said the whole issue was resolved by the EPA back in 2006, when it determined that Atrazine was perfectly safe.

Prof. HAYES: If the issue was resolved by the EPA, then the EPA wouldn't be holding three scientific advisory panels this year to review Atrazine. So I think what you're hearing from Tim Pastoor, as they say, the money talking, not the science.

RAZ: Syngenta also says that farmers in the U.S. are worried about these kinds of studies because they believe they are unfairly alarmist and that it could lead to a ban on this herbicide here, which would then have a serious impact on the agriculture industry.

Prof. HAYES: Well, Atrazine has been banned in Europe, and studies have been done, both in corn production in Europe and on their economy. The European economy did not collapse in the absence of Atrazine, and they're still growing corn.

RAZ: So, now where do you go from here? I mean, what additional work will you be doing looking into the effect of this, or possible effect of this herbicide on other species?

Prof. HAYES: We're not focusing so much on other species right now. We're in the middle of publishing about six long-term studies. This is the first of several. One of the next studies to come out will focus more on male-male copulation with males that aren't transformed into females but still mate with other males. And we're moving on from there to try to understand the brain differentiation and how hormones affect the brain, then the behavior that would tell an animal whether or not it should perform as a male or a female.

RAZ: That's Tyrone Hayes. He's a professor of biology at U.C. Berkeley. His study on Atrazine and frogs can be found in the new issue of the Proceedings of the National Academy of Sciences.

Tyrone Hayes, thank you so much.

Prof. HAYES: Thank you.

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