18 April 2002

Weed Killer Disrupts Frogs' Sexual Development

by Kate Melville

The nation's top-selling weed killer, atrazine, disrupts the sexual development of frogs at concentrations 30 times lower than levels allowed by the Environmental Protection Agency (EPA), raising concerns about heavy use of the herbicide on corn, soybeans and other crops in the Midwest and around the world.

In the April 16 issue of Proceedings of the National Academy of Sciences, developmental endocrinologist Tyrone Hayes and colleagues from the University of California, Berkeley, report that atrazine at levels often found in the environment demasculinizes tadpoles and turns them into hermaphrodites creatures with both male and female sexual characteristics. The herbicide also lowers levels of the male hormone testosterone in sexually mature male frogs by a factor of 10, to levels lower than those in normal female frogs. The research was funded by the National Science Foundation (NSF).

"Researchers are investigating a number of possible causes for declining populations of amphibians," said William Zamer, program director in NSF's division of integrative biology and neuroscience, which funded the research. "Hayes and colleagues appear to have identified another potential agent. Their findings indicate that atrazine has significant endocrinological effects on frogs at very low doses."

As Hayes discovered, many atrazine-contaminated ponds in the Midwest contain native leopard frogs with the same abnormalities. "Atrazine-exposed frogs don't have normal reproductive systems," he said. "The males have ovaries in their testes and much smaller vocal organs," which are essential in calling potential mates. It is unclear whether these abnormalities lead to reduced fertility. Hayes now is trying to determine how the abnormalities affect the frogs' ability to produce offspring.

"The use of atrazine in the environment is basically an uncontrolled experiment - there seems to be no atrazine-free environment," Hayes said. "Because it is so widespread, aquatic environments are at risk." Because the herbicide has been in use for 40 years in some 80 countries, its effect on sexual development in male frogs could be one of many factors in the global decline of amphibians, he added.

The findings come at a time when the EPA is re-evaluating allowable levels of atrazine in drinking water, which stand today at 3 parts per billion (ppb), and has drafted new criteria for the protection of aquatic life, limiting four-day average exposures to 12 ppb. Hayes found hermaphroditism in frogs at levels as low as 0.1 ppb. Even with today's limits, levels of 40 ppb atrazine have been measured in rain and spring water in parts of the Midwest, while atrazine in agricultural runoff can be present at several parts per million.

The herbicide also contaminates drinking water supplies in many communities in the Midwest, leading some environmental groups to voice concern about its effect on children, infants and the fetus. France, Germany, Italy, Sweden and Norway are among countries that have banned the use of atrazine.

To date, atrazine's effects on mammals and amphibians have been tested only at large doses, not at doses commonly found in the environment.

In their journal article, Hayes and his colleagues write, "the effective doses in the current study ... demonstrate the sensitivity of amphibians relative to other taxa, validate the use of amphibians as sensitive environmental monitors/sentinels, and raise real concern for amphibians in the wild." Hayes doubts that atrazine has such severe effects on humans, because the herbicide does not accumulate in tissue and humans don't spend their lives in water like frogs do.

Nevertheless, the effects of atrazine on frogs could be a sign that the herbicide is subtly affecting human sex hormones, too, interfering with androgens, such as testosterone, that control male sex characteristics.