Can there be a causal connection between the following two circumstances?
(1) Since the 1980s, in many parts of the world from North America to Australia the populations of amphibians, especially frogs, have been plunging, for no known reason.
(2) The most heavily used weed-killer in the U.S. and probably the world is a chemical called atrazine.
Farmers have dosed their growing crops for over 40 years with this herbicide. Manufactured by Geneva-based Syngenta AG, atrazine is currently applied in more than 80 countries around the globe. The U.S. Department of Agriculture reports that more than 30 thousand tons 60 million pounds of it are used annually in the U.S. alone. And the Environmental Protection Agency sets the allowable concentration of atrazine in drinking water at 3 parts per billion.
Atrazine disrupts the sexual development of frogs at concentrations 30 times lower than levels allowed by EPA. This raises concerns among farmers in the Midwest and around the world about its intensive use on corn, soybeans and other crops. The herbicide also contaminates drinking water in many communities in the Midwest, leading some environmental groups to worry about atrazine’s effects on children, infants and the fetus. France, Germany, Italy, Sweden and Norway have banned its use.
“The use of atrazine in the environment is basically an uncontrolled experiment,” observed amphibious developmental endocrinologist Tyrone Hayes at the University of California at Berkeley. “There seems to be no atrazine-free environment,” he added, “and because it is so widespread, aquatic environments are at risk.”
Hayes is senior author of a paper in the current Proceedings of the National Academy of Sciences (PNAS) dated April 16, 2002. Its title raises the weed-killer/frog-nemesis puzzle: “Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses.”
“Our overall findings,” Hayes told BioWorld Today, “are that atrazine, at surprisingly low dosage levels, produced hermaphrodites in the African clawed frog [Xenopus laevis]. These were the results of demasculization and feminization. We believe that the lack of growth of the larynx in proper male fashion was the result of a decrease in testosterone, and that subsequent to that, the development of gonadal abnormalities, including both ovaries and testes in male animals, resulted from the production of estrogen.”
Hormonal Unisex Villain: Aromatase Enzyme
For atrazine’s putative anti-frog molecular mode of action, Hayes cited work by zoologist John Giesy at Michigan State University in East Lansing: “He found last year that, at large doses, atrazine ups production of the enzyme aromatase, which converts male androgen hormones to estrogen hormones. The lowered testosterones,” Hayes went on, “would interfere with the male frogs’ laryngeal voice-box development, while increased estrogen would promote ovaries within the testes.”
In their own in vivo experiments, Hayes and his co-authors suspected that atrazine produced small, squeaky voices, rather than the full-throated croaking of normal male frogs, which is essential for luring potential female mates. ”We believe that’s due to atrazine’s induction of aromatase,” Hayes said, “the enzyme that converts testosterone to estrogen.”
In their experiments, he recounted, “We exposed African clawed frogs as larvae from hatching to metamorphosis at several doses of atrazine, ranging from .01 parts per billion to 200 parts per billion. That dose range encompassed what’s considered safe by the EPA in drinking water. It is ecologicallally relevant, as it represents atrazine concentrations that can be found in everything from streams to ponds to lakes to agricultural runoffs, to snow runoff and drinking water.
“We immersed the tadpoles,” Hayes continued, “with atrazine dissolved in their water, and they were exposed continuously to the multiple doses. The gonadal disabilities and deformities resulting in the lab,” he pointed out, “have not been found in the wild in dead frogs. We did some recent analysis in the field, which hasn’t been peer-reviewed yet,” he said. “I can say that the abnormalities are not restricted to African clawed frogs, nor their response to laboratory dosing.
“These disabilities, the larynx, the gonads and so on,” he noted, “are not life threatening. There are no generalized health or growth or developmental effects. The frogs look perfectly fine, except for the fact that the males have ovaries in addition to their testes. And we also looked at the ability of these impaired male animals to fertilize females. These experiments are completed, and we are preparing the studies right now for publication.”
Hence, the global amphibian population fall-off is not akin to a massacre. “If you’re not reproductively functional,” Hayes noted, “then the population declines. So if you are a male that makes eggs instead of sperm, you’re unable to carry the population and keep it growing. Those individuals don’t suffer death. But by being reproductively impaired, they’re unable to maintain the population.”
Blame Decline On Multiple Factors, Not One
Nor has the jury rendered a verdict as to atrazine’s guilt, if any, in the population crash. “We really don’t have enough data right now to tie it to atrazine, or any one other factor,” Hayes said. “Even if I felt that we could make that link, I would argue that atrazine is not the only compound that would interfere with amphibian development. Even if we could link the decline to chemicals, I would argue that it really isn’t just chemicals. There’s a number of interactions possibly predators that can contribute to amphibian declines. Now that we have enough data, people don’t doubt any more that we’re losing biodiversity with regard to frogs.
“Right now we’re doing comparative studies on other species, and fieldwork as to the effects on leopard frogs [Rana pipiens] and Pacific tree frogs [Hyla regilla] exposed in the wild. Also, we have several collaborations,” Hayes concluded, “to look at the biochemical and molecular mechanisms.”
“This is very important and elegant work,” commented Theo Colburn, a senior scientist at the World Wildlife Fund. He is an internationally recognized expert on endocrine-disrupting chemicals. “Hayes’ work demonstrates,” Colburn observed, “the need to do research on the safety of chemicals in the field. The changes he found in the gonads were not discovered with the traditional high-dose atrazine experiments of the past. In addition, microscopic examination of the frogs’ internal organs is required to detect the hidden effects from low-dose exposure.”