One of the world’s most popular pesticides, atrazine, chemically castrates male frogs and in some instances changes them into completely functionally females, according to a new study in the Proceedings of the National Academy of Sciences. The authors conclude that atrazine likely plays a large, but unsuspected role in the current global amphibian crisis.

To study how atrazine impacts frogs, researchers studied the long-term effects of the pesticide on an all-male group forty of African clawed frogs (Xenopus laevis).

Even though they exposed the frogs to a low doze (2.5 parts per billion, i.e. ppb) of the pesticide, the researchers found that 10 percent of the animals became full-fledged females due to exposure to the pesticide, capably even of breeding successfully with males. The other 90 percent of the exposed males were demasculinized, suffering from decreased testosterone, low fertility, and an inability to outcompete non-exposed males in breeding.



Two males copulating. The larger animal on the bottom (though genetically male) has been completely feminized by atrazine exposure and produces viable eggs. Image courtesy of Tyrone B. Hayes.



“The impacts of atrazine on amphibians and on wildlife in general are potentially devastating,” the authors write. “The negative impacts on wild amphibians is especially concerning given that the dose examined here (2.5 ppb) is in the range that animals experience year-round in areas where atrazine is used, well within levels found in rainfall, in which levels can exceed 100 ppb in the Midwestern United States, and below the current US Environmental Protection Agency drinking water standard of 3 ppb.”

According to the paper: “atrazine is the most commonly detected pesticide contaminant of ground, surface, and drinking water” due to its popularity. Eighty million pounds are used annually in the United States alone with rainfall spreading the pesticide far beyond its application point.

Worldwide amphibians are in decline: at least one third of the world’s amphibians are currently threatened with extinction, and researchers estimate that over 120 amphibian species have gone extinct in just thirty years. While scientists have paid a lot of attention to particular threats such as habitat loss, climate change, and a fungal disease that is known to wipe-out whole amphibian populations, far less attention has been given to the possible role of pesticides, like atrazine.

“The present study suggests several ways that exposure to endocrine disruptors such as atrazine may lead to population level effects in the wild and contribute to amphibian declines,” the researchers write. “Certainly, the inability to compete for females and the significant decline in fertility in exposed males, as reported in the present study, will have a direct impact on exposed populations.”

In addition, male frogs that become wholly female due to the pesticide can only in-turn produce male young. The researchers say that mathematic modeling proves such a situation could lead to the eventual extinction of an entire species.

But it’s not just amphibian survival that is at stake, past studies have shown atrazine to be a potent endocrine disrupter in fish, reptiles, birds, rodents, and even human cell lines.

Atrazine was banned in the EU in 2004, but remains widely used in the US and around the world.

Citation: Tyrone B. Hayes, Vicky Khoury, Anne Narayan, Mariam Nazir, Andrew Park, Travis Brown, Lillian Adame, Elton Chan, Daniel Buchholz, Theresa Stueve, and Sherrie Gallipeau. Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). PNAS.

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