Herbicide makes frogs Mr. Moms: hormonal tinkering may explain a host of adverse changes.
Two studies have fingered a potential contributor to the widespread decline of amphibians: water contaminated with atrazine, a weed killer used widely on corn, cotton and turf. In the lab, atrazine fully feminized some male frogs. Outdoors, in pools designed to simulate ponds, the herbicide drastically reduced the share of wild tadpoles able to metamorphose.
Atrazine concentrations triggering these changes--up to 2.5 parts per billion--regularly occur in North American ponds and streams. The Environmental Protection Agency allows up to 3 ppb of atrazine in drinking water.
Tyrone Hayes and his colleagues at the University of California, Berkeley, exposed male frogs in the lab to 2.5 ppb of atrazine. These animals developed few sperm and had very low testosterone levels. More provocatively, 10 percent of these males transformed into functional "females" that encouraged the advances of healthy males, and in two cases were found to have produced eggs that hatched into viable young.
The Berkeley group reported its findings online March 1 in the Proceedings of the National Aeademy of Sciences.
Separately, University of Ottawa scientists teamed up with colleagues at two federal agencies in Canada to study atrazine's effects on northern leopard frogs. The team collected fertilized eggs from local ponds, incubated them until hatching, and then transplanted the tadpoles into 378-liter outdoor pools.
The brains of animals exposed to higher atrazine levels saw a 2.5-fold increase in receptors for estrogen. This change could render the animals more susceptible to the feminizing effects of their own estrogen or environmental mimics of this hormone, notes Vance Trudeau, one of the Ottawa scientists.
Higher exposure also altered activity of a liver enzyme that converts testosterone into another male sex hormone. And it perturbed production of thyroid hormones that play a pivotal role in metamorphosis, Trudeau's team reported online recently in Environmental Health Perspectives.
In fact, atrazine-exposed pollywogs proved only about half as likely to metamorphose as those raised in clean water. "I would consider this quite important," Trudeau says, "because if you have 50 percent less metamorphosing, you're likely to have more [young] picked off" by predators.
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|Date:||Mar 27, 2010|
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