Common Weed Killer Makes Male Frogs Lay Eggs

Two frogs. The larger animal on the bottom (though genetically male) has been completely feminized by atrazine exposure and produces viable eggs.

A healthy male frog (bottom) mates with a male turned female by exposure to the chemical atrazine.

Photograph courtesy Tyrone B. Hayes

Rachel Kaufman

Published March 1, 2010

The so-called pregnant man has company: One of the most common weed killers in the United States can make male frogs lay eggs, a new study says.

Atrazine, widely used to kill pests on U.S. croplands, is an endocrine disruptor—a substance that interferes with animals' reproductive systems.

Previous research has shown that atrazine can give male amphibians female characteristics: For instance, male frogs exposed to atrazine have lower testosterone levels, produce less sperm, and even change their mating habits by choosing males over females.

Now, researchers have discovered that the chemical transforms male frogs into fully functioning females—and that the substance may be contributing to a worldwide decline in amphibians.

Atrazine "Keeps Coming Back"

To test the chemical's impact, study leader Tyrone Hayes raised 40 genetically male African clawed frogs from hatching to adulthood in a solution of 0.003 ethanol, which contained 2.5 parts per billion of atrazine.

Four of the adult frogs—or ten percent—developed into what looked like perfectly normal females.

Hayes, a biologist at the University of California, Berkeley, and his team dissected two of the four and confirmed that they had ovaries but had maintained their male DNA.

The other two frogs mated with males and laid eggs that hatched and grew to adulthood. All the offspring were chromosomally male and seemingly healthy. (Learn how DNA works.)

Of the atrazine-exposed males that didn't develop ovaries, 80 percent were unable to produce sperm.

Hayes, also a National Geographic Society emerging explorer, said that even he was somewhat surprised at the myriad effects of the herbicide. (The National Geographic Society owns National Geographic News.)

"As a scientist, you always think there's got to be an alternate explanation," he said. "But the thing about atrazine is it just keeps coming back."

Human Impacts?

Though there haven't been many studies on the chemicals' impacts on people, some recent research has linked atrazine exposure to breast cancer.

The U.S. Environmental Protection Agency recently announced it's reevaluating atrazine—banned in the European Union since 2004—to determine possible harmful effects to people.

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