The most heavily used herbicide in the United States, atrazine, makes hermaphrodites of male frogs at concentrations commonly found in the environment, according to a study published in the 16 April issue of the Proceedings of the National Academy of Sciences. The finding may help explain why amphibian populations are rapidly disappearing, experts say, and could influence debate over atrazine use in the United States.
For over a decade, scientists have watched with alarm as amphibian populations have mysteriously declined. Some species have gone extinct. One suggested cause has been pesticide contamination. Although some 27 million kilograms of atrazine are used annually in the United States, toxicologists had come to regard it as relatively benign, with effects only at abnormally high levels. But researchers had never tested its effects on an apparent amphibian Achilles' heel: the hormone system.
Now, developmental endocrinologist Tyrone Hayes and colleagues at the University of California, Berkeley, have raised tadpoles of the African clawed frog, Xenopus laevis--the lab rat of the amphibian world--in water with levels of atrazine varying from 0.01 to 200 parts per billion (ppb). At and above 0.1 ppb, they found that 16% to 20% of the animals developed up to six gonads, including both testes and ovaries. In male adult frogs exposed to 25 ppb of atrazine, testosterone levels dropped 10-fold, to levels found in females.
Although the Environmental Protection Agency's (EPA's) current safe drinking water standard is 3 ppb, atrazine is often found in central North American waters at concentrations of up 10 ppb in spring, and peaks greater than 200 ppb have been recorded. Because Hayes and colleagues found no effects on mortality or external appearance, they argue that endocrine abnormalities might easily occur in the wild and never be noticed.
"This study is groundbreaking, there's no doubt about it," says Val Beasley, an ecotoxicologist at the University of Illinois, Urbana-Champaign. While amphibian declines likely have multiple interacting causes, adds James Collins of Arizona State University in Tempe, this research may well "give us an important piece of the puzzle."
On 16 April, the EPA will release its preliminary risk assessment for atrazine at a technical briefing, a key step in its long process of reviewing the chemical's effects on humans, wildlife, and the environment. Atrazine is banned in many European countries, and some observers expect the study may help lead the EPA to become tough on the compound as well.