The top-selling weed killer in the United States, atrazine, has been found to disrupt the sexual development of frogs at concentrations 30 times lower than levels allowed by the Environmental Protection Agency (EPA). This finding by developmental endocrinologist Tyrone B. Hayes, associate professor of integrative biology at the University of California, has raised concerns about heavy use of the herbicide on corn, soybeans and other crops in the Midwest U.S. and around the world.
The study reports 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. Hayes discovered that many atrazine-contaminated ponds in the Midwest contain leopard frogs with these 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.
"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."
The herbicide has been in use for 40 years in some 80 countries, so its effect on sexual development in male frogs could be one of many factors in the global decline of amphibians, he added.
More than 60 million pounds of the herbicide were applied last year in the United States. France, Germany, Italy, Sweden and Norway are among countries that have banned the use of atrazine.
In laboratory experiments at various concentrations of atrazine, using two separate populations of frogs raised in three separate tanks, atrazine was found to affect the sexual development of frogs at concentrations of 0.1 ppb and higher. That is 30 times lower than the allowable EPA limit of 3 ppb in drinking water and 120 times lower than the proposed chronic exposure limit for aquatic life, 12 ppb.
At these concentrations, as many as 16 percent of the animals had more than the normal numbers of gonads - including one animal with six testes - or had both male and female organs. No control animal had such abnormalities.
Another study in the western U.S. has found a more indirect relationship between chemicals and abnormalities in amphibian populations. This unproven theory is based on the belief that nutrient pollution from fertilizers and cattle may increase the numbers of snails and parasites which in turn may increase deformities in amphibians. In recent years, the frequency of frogs, toads, salamanders and other amphibians with missing limbs, extra limbs, and other deformities has increased.
In a field survey headed by Pieter Johnson of the University of Wisconsin-Madison covering parts of California, Oregon, Washington, Idaho, and Montana, researchers looked for malformations in over 12,000 amphibians of 11 different species. The group looked at the relationships between the frequency and severity of abnormalities and a variety of factors including the abundance of the parasite ribeiroia and pesticide contamination.
They found malformed amphibians at a wide variety of sites. While the researchers did not find a relationship between pesticides and the frequency of malformed amphibians, they did find a striking connection between malformed amphibians and the presence of ribeiroia.
"The presence of this parasite was a powerful predictor of the presence and frequency of malformed amphibians in an aquatic system", said Johnson. The greater an amphibian population's infection with ribeiroia, the more frequent and severe the population's limb malformations.
"People assume that parasites are 'natural' and therefore of no conservation concern. However, we suspect that nutrient pollution from fertilizers and cattle may be increasing the numbers of snails, parasites, and therefore malformed amphibians."