Popular weed killer de-masculinizes frogs, disrupts their sexual development

Public release date: 15-Apr-2002

Brenda Lana Smith R.af D.

Berkeley - 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 an article in the April 16 issue of Proceedings of the National Academy of Sciences, University of California, Berkeley, developmental endocrinologist Tyrone B. Hayes, associate professor of integrative biology, and his colleagues 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.

As Hayes later 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.

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.

"This is very important and elegant work," said Theo Colborn, PhD, a senior scientist at the World Wildlife Fund and an internationally recognized expert on endocrine disrupting chemicals. "Tyrone's work demonstrates the need to do research on the safety of chemicals in the field where the animals live and at the levels to which they are exposed. The changes he found in the gonads were not discovered with the traditional high-dose atrazine experiments used in the past.

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 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. Extrapolating these results from mammalian cells to amphibians, Hayes argues that atrazine could feminize male frogs by promoting the conversion of male hormones to female hormones. The lowered androgens would interfere with voice box development, while increased estrogens would promote ovaries within the testes. The studies were supported by the National Science Foundation.