Feminization blamed on farming: agricultural chemicals suspected in cane toad troubles.

Among toads living in farm country, gents tend to resemble the gals--both inside and out. This doesn't bode well for the hoppers impressing local ladies, much less fathering their tadpoles.

Toads and other amphibians throughout the world are under siege. Disease plagues many populations; others suffer high rates of deformities or immune suppression. In some regions, species have simply vanished without a trace.

To investigate whether these problems stem from general stress due to land development, rather than poisoning by chemicals, University of Florida biologists analyzed dozens of local adult cane toads (Bufo marinus). At least 20 were collected during summer nights at each of five sites in Florida. Some toads had been living in suburbs, others near anything from a little farming to heavy agriculture. As hectares of farming in the toads' vicinities increased, so did the proportion of males exhibiting a serious feminization, Krista McCoy and colleagues report in an upcoming Environmental Health Perspectives.

If just land disturbance--to build roads, homes, shopping malls and the like--was a sufficient stressor, the effects on toads living near suburbs should be no different than for toads living near farms. That there was a difference suggests that something about farming is to blame, such as the chemicals used on farms--many of which in isolation can cause feminization of amphibians and other animals.

At sites where half to 97 percent of nearby land was farmed, about 40 percent of the 67 non-females had typical he-toad gonads and coloring. An equal number were intersex animals, with testes and ovaries. About 20 percent of the non-females near the highest farming activity appeared outwardly male. But these superficially normal males sported a maturing Bidder's organ, as did the intersex animals.

Like the human appendix, the Bidder's organ normally has no function. But if males lose testicular function, it may mature into an ovary, says wildlife endocrinologist Louis Guillette Jr., a coauthor on the study. His team found that Bidder's organs in some of the males from the farming regions were full of eggs (although the viability of the eggs was not checked).

Feminized male toads had a female coloration, shorter forearms than normal males and fewer nuptial pads (temporary features that develop on the fingers of males who are readying to mate). Levels of testosterone, the primary macho sex hormone, were especially low in male toads from sites near substantial farming.

"I've worked with this species," says David Crews of the University of Texas at Austin, and "these are about the toughest amphibians in the world." So he was impressed by the new data, which suggest that something about nearby farming--probably runoff of pesticides or other chemicals--is "essentially remaking the individual," inappropriately "resculpting" a male tadpole's urogenital tract.

Lab studies by Tyrone Hayes of the University of California, Berkeley show that the types of gross demasculinizing changes reported in the new study would make animals sterile. Hayes has investigated gonadal changes in frogs exposed to agricultural chemicals, such as atrazine--a weed killer used by farmers raising sugarcane, a primary crop in the Florida settings where toads were sampled.

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But malformed organs or a feminine appearance aren't the only mating obstacles, Hayes notes. Chemical exposures also affect behavior. "Out of every trial we've done," he says, "only two atrazine-treated males were ever able to even copulate."

Although the Florida team does not speculate on what in the tadpoles' water feminized them, Hayes says that a primary suspect has to be "atrazine, obviously." Any reproductive remodeling of the male toads' bodies probably also traces back to additional chemicals, he says.

The study area included suburban, somewhat agricultural and very agricultural land, allowing the scientists to identify "dose-dependent" effects of farming on internal organs, hormones and even outward appearance, observes Pamela Martin of Environment Canada in Burlington, Ontario. These changes "corroborated each other very well," she adds, "making for a very convincing story."

A paper by Martin's team in the July 30 Aquatic Toxicology makes a similar link between agricultural intensity and the feminization of male amphibians--in this case northern leopard frogs.