ATHENS, Ohio -- Pesticides can mimic the properties of reproductive hormones in frogs and turtles, and global pollution levels of such chemical compounds may play a significant role in disappearing amphibian populations, according to an article published in the May edition of Environmental Health Perspectives.
"It's the first time it's been shown in a laboratory that these chemical compounds are acting as estrogens in amphibians, although there has been speculation in the past," said Palmer, whose article, "Vitellogenin Induction by Xenobiotic Estrogens in the Red-eared Turtle and African Clawed Frog," appeared in the monthly publication of the National Institutes of Health. "Certainly this could be a significant nail in the coffin for amphibian populations."
Scientists have been puzzled for decades over the loss of many amphibian populations, such as the northern leopard frog, the Western Toad and the Cascades Frog in North America. It is believed that 95 percent of the once widespread leopard frog population died off in a crash that began in the Eastern United States in the 1960s and continued westward.
Habitat loss, increased ultraviolet radiation from ozone depletion and acid rain have all been studied as the potential smoking gun in various declining amphibian populations, but pesticide pollution may provide another factor which is present across the globe.
"There are no animals in the world that don't have measurable levels of DDT and dioxins -- regardless of how far away they are from pesticide use or industrial pollution," Palmer said.
Populations of red-eared turtles and African clawed frogs were, respectively, injected with DDT, DES (an anti-abortive compound used from the 1940s until the 1970s), and estrogen in Palmer's study.
Males injected with the chemicals all produced significant amounts of vitellogenin, a protein induced by the female hormone, estrogen. Vitellogenin is the precursor of egg yolk when present in ovaries, and no male hormones should produce this protein.
"Males, although capable of producing vitellogenin, don't do it because of the lack of estrogen," Palmer said. "Essentially, males are being feminized -- their bodies are trying to produce egg yolk."
The amount of vitellogenin produced by the varying chemical levels was measured by a color density produced in a mixture of blood samples and select antibodies. The color change is actually induced by the response of the antibodies to a particular concentration of vitellogenin.
Estrogen was injected in a solution of one part per million, DES at one part per million and DDT at both one part per million and 250 parts per million in different groups of both turtles and frogs.
"What interested me is we were still getting a response from such a low dosage (one part per million) of DDT," Palmer said. "We have no idea of what the minimum dosage (to produce hormonal or endocrinal disruption) may be."
Injections of one part per million of DDT produced vitellogenin expression about one-tenth as strong as that of estrogen itself, Palmer noted. At one part per million, the expression from DDT was anywhere from one-third to one-half as strong as the expression of DDT at 250 parts per million.
Although DDT was banned by the United States in 1972, background levels continue to rise through use in other countries, such as Mexico. Turtles are often found to have DDT concentrations of as much as 800 parts per million in blood and tissue, and frogs as much as 200 parts per million.
DES, used for decades to prevent miscarriages, was included in the testing because of its well-documented effect on the reproductive tracts of children produced by women using the drug. Both male and female offspring of these women were found to have reproductive problems, including cervical and breast cancer, in past DES testing.
In Palmer's study, DES produced vitellogenin expression that was only slightly less strong than that of estrogen. "In the case of DES, the effects weren't seen until 20 years down the road, where the children were found to be barren or have reproductive problems. That's what is so frightening, because you don't see the effect on people right away," he said.
Chemical pollutants such as pesticides may be having an effect on both male and female populations in the wild, Palmer said. "DDT is actually a relatively weak estrogen, but it is extremely common and doesn't go away."
Most animals are believed to accumulate xenobiotic estrogen through the food chain, but the skin of amphibians is highly permeable and could also expose them through both water and air pollution. Palmer believes his vitellogenin tests can now be used as a marker to determine the exposure of wildlife to these xenobiotic estrogens or environmental endocrine disrupters.
"Both males and females may be affected," he noted. "Thirty percent of breast cancers (in women) are estrogenic, and the fat cells of the breast are very efficient at collecting pesticides such as DDT."