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Posted by desiree on November 05, 2002 at 10:40:23:
Herbicide makes wild frogs hermaphrodite
Atrazine gives frogs male and female gonads, says field study.
31 October 2002
Frogs are in decline worldwide.
A widely used herbicide is making male frogs grow female gonads in the US midwest, according to a recent field study. The finding could fuel the controversy over whether or not the chemical is one of the many possible reasons amphibian populations are shrinking worldwide.
Atrazine seems to make leopard frogs (Rana pipiens) hermaphrodite in the wild as well as in the lab1,2. "Leopard frogs are the most common, widely distributed species of native American frog," points out study leader Tyrone Hayes of the University of California, Berkeley.
US farmers use about 27,000 tonnes of atrazine each year to protect maize and other crops from weeds. So the compound gets into rain, groundwater, rivers and streams. The US Environmental Protection Agency has set a standard of 3 parts per billion (p.p.b.) as an acceptable level in drinking water. But concerns about atrazine's ability to disrupt sex hormones have led many European nations to ban it.
Hayes and his colleagues checked frogs and atrazine levels at eight sites from Iowa to Utah. They found that up to 92% of male frogs at sites contaminated with atrazine had abnormal gonads. In the lab, almost a third of leopard frog tadpoles developed mixed gonads when exposed to 0.1 p.p.b. atrazine, well below typical environmental levels. The same happened with male African clawed frogs (Xenopus laevis)3.
But whether or not atrazine is really the cause of the abnormalities seen and if so, how, is hotly debated.
James Carr, who studies the effects of pollutants on amphibian reproduction at Texas Tech University in Lubbock, says that he does not see the same effects as Hayes. In the lab, Carr only sees a significant number of frogs with mixed sex gonads at atrazine levels of 25 p.p.b. - much higher than environmental levels.
"We're not saying that atrazine does not cause gonadal abnormalities," says Carr. "But we're not seeing it at the same threshold level as Hayes." Carr's work is funded by Syngenta, the company based in Basel, Switzerland, that manufactures atrazine.
Other frog researchers side with Hayes, seeing his work as an important link between agricultural chemicals and declining amphibian numbers. "I don't have any doubt that his conclusions are sound and results are valid," says herpetologist James Hanken of Harvard University in Cambridge, Massachusetts.
"I see this as a second Silent Spring," Hanken says, referring to the book on pollution's toll on birds. "Now we're documenting another whole round of pollutant effects on amphibians."
Hanken says there are several "smoking guns" in the worldwide amphibian slump, including increased exposure to ultraviolet radiation, habitat erosion, trade and now pollution. Amphibians may be sensitive to environmental changes, he suggests, because they are aquatic and terrestrial, and because they absorb water easily through their porous skin.
Hayes, T., Haston, K., Tsui, M., Hoang, A., Haeffele, C. & Vonk, A. Feminization of male frogs in the wild. Nature, 419, 895 - 896, (2002). |Article|
Hayes, T. et al. Atrazine-Induced Hermaphroditism at 0.1 PPB in American Leopard Frogs (Rana Pipiens): Laboratory and Field Evidence. Environmental Health Perspectives, published online doi:10.1289/ehp.5932 (2002). |Article|
Hayes, T. et al. Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses. Proceedings of the National Academy of Sciences, 99, 5476 - 5480, (2002). |Article|
© Nature News Service / Macmillan Magazines Ltd 2002