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Feminized Frogs: Herbicide Disrupts Sexual Growth

Ben Harder, Science News, 161(16): 243 04/20/2002


At common environmental concentrations, the popular weed killer atrazine strips male frogs of a key hormone and turns some of them into hermaphrodites, according to new research. The finding raises concerns that the chemical may be contributing to global amphibian declines.

In use for about 4 decades and currently employed in 80 countries atrazine is the most common herbicide in the United States. It's found in virtually all the nation's waterways and is especially prevalent around cornfields in the Midwest. It has also been identified in tests of preschoolers' drinking water (SN: 11/3/01, p. 285: Available to subscribers at "There seems to be no atrazine-free environment," says Tyrone B. Hayes of the University of California, Berkeley.

Past research has found no evidence that typical environmental concentrations of atrazine cause premature death or abnormal growth in amphibians. The new research, which Hayes and his colleagues report in the April 16 Proceedings of the National Academy of Sciences, similarly finds that mortality and overall growth of the common lab frog Xenopus laevis are unaffected by atrazine.

However, the researchers report, the herbicide has significant effects on frogs' sexual development. These turn up at concentrations substantially lower than the 3 parts per billion (ppb) that the Environmental Protection Agency permits in drinking water.

To investigate atrazine's effects on sexual development, the researchers exposed tadpoles to concentrations ranging from 0.01 to 200 ppb. At concentrations of 0.1 ppb or above, 16 to 20 percent of the males developed extra testes or even ovaries. Concentrations of the male sex hormone testosterone in the blood of adult males exposed to atrazine were one-tenth those in blood from unexposed males. Exposed males also developed smaller larynxes, organs that are important for frogs' sexual communication.

By looking for more subtle effects than past studies examined, Hayes and his team have identified an important environmental problem, says William H. Karasov of the University of Wisconsin–Madison. Because the UC-Berkeley study found atrazine acting on frogs at "ecologically relevant" water concentrations, it's imperative that scientists gather data on the sexual development of amphibians living in the wild, says Karasov.

These findings are "going to really shake a lot of people up," says Thomas W. La Point of the University of North Texas in Denton. Atrazine's effects "could very well explain a good portion of why [amphibians] are declining," he adds.

Work in other labs supports this concern. A recent Canadian study, reported in the March Environmental Toxicology and Chemistry, finds that atrazine impairs testes development in X. laevis.

Research by Warren P. Porter of the University of Wisconsin–Madison indicates that atrazine also disrupts other hormonal systems, he says.



Hayes, T.B., et al. 2002. Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses. Proceedings of the National Academy of Sciences 99(April 16):5476-5480. Abstract.

Tavera-Mendoza, L., et al. 2002. Response of the amphibian tadpole (Xenopus laevis) to atrazine during sexual differentation of the testis. Environmental Toxicology and Chemistry 21(March):527-531. Abstract.

Further Readings:

Milius, S. 2000. Colossal study shows amphibian woes. Science News 157(April 15):247.

Mlot, C. 1997. Water link to frog deformities strengthened. Science News 152(Oct. 11):230.

Raloff, J. 2001. How polluted is a preschooler's world? Science News 160(Nov. 3):285.

______. 1999. Thyroid linked to some frog defects. Science News 156(Oct. 2):212.

______. 1998. Common pesticide clobbers amphibians. Science News 154(Sept. 5):150.


Tyrone B. Hayes
Laboratory for Integrative Studies in Amphibian Biology
Group in Endocrinology
Museum of Vertebrate Zoology
Department of Integrative Biology
University of California, Berkeley
Berkeley, CA 94720

William H. Karasov
Department of Natural Resources - Wildlife Ecology
University of Wisconsin, Madison
Madison, WI 53706

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