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When frog gender flips

With weed killer in the water, some males acquire female traits.

Several months ago, a California college student working at a university laboratory checked up on a group of frogs and saw some unusual behavior. Some of the frogs were acting like females — which was unusual, since at the beginning of the experiment all of the frogs were males.

The student, Ngoc Mai Nguyen, says she told her boss, biologist Tyrone Hayes, “‘I don’t know what’s going on, but I don’t think this is normal.'” Nguyen, a student at the University of California, Berkeley, was working in Hayes’ laboratory. Hayes told Nguyen to keep watching — and write down what she saw each day.

Nguyen knew all the frogs had started out as males. She didn’t know, however, what Hayes knew: that there was something in the water of the frog tank. That something was a popular weed killer called atrazine, and since birth the frogs had been raised in water that contained the chemical.

Hayes says the experiments in his lab show that 30 percent of the male frogs that grew up in water with atrazine started to behave like females, and even send out chemical signals to attract other males.

When this frog species is raised in the lab in water tainted with what EPA considers acceptable concentrations of atrazine, males change — sometimes into apparent females.

When this frog species is raised in the lab in water tainted with what EPA considers acceptable concentrations of atrazine, males change — sometimes into apparent females.

Furryscaly/Flickr

Laboratory experiments are not the only places where frogs may run into atrazine. The chemical is used as a weed killer, so it can pollute surface water downstream of the crops where it is used. In these rivers and streams, the concentration of atrazine can reach 2.5 parts per billion — the same concentration Hayes tested in his laboratory. This similarity suggests that male frogs may be turning into females in their natural habitats.

The Environmental Protection Agency, or EPA, is a government organization responsible for protecting human health and the environment. The EPA defines what concentrations of certain chemicals are allowed in U.S. waterways, and the EPA has determined that up to 3 parts per billion — well above the concentration that turns male frogs into females — of atrazine is safe. If Hayes is right, even the EPA definition of a safe concentration is actually not safe for frogs.

Hayes and his team have also showed that it’s not just the frogs’ behavior that changes after exposure to atrazine. Males raised in water containing atrazine had low levels of testosterone and did not try to attract females.

But that’s not all: Out of 40 frogs raised in water containing atrazine, four had high levels of estrogen — a female hormone (that’s four out of 40 frogs, or one in 10). Hayes and his team dissected two of the frogs and found female reproductive organs. The other two transgender frogs were introduced to healthy males and mated with those males, producing baby male frogs.

Other scientists have looked at Hayes’ work and carried out similar experiments — with similar results. Plus, researchers who study other animals have observed that atrazine affects those animals’ hormones.

At least one scientist, Tim Pastoor, says Hayes has made mistakes in his study and that atrazine is safe. Pastoor is a scientist with Syngenta Crop Protection, a company that makes and sells atrazine.

In an email to Science News, Pastoor wrote that Hayes’ new experiments don’t lead to the same results as Hayes’ earlier studies. “Either his current study discredits his previous work, or his previous work discredits this study,” Pastoor wrote.

It’s important to know how atrazine affects the animal population. Any chemical that can change the reproductive patterns of an animal threatens that species’ survival.

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