Amphibians

As far as I am aware, presently there is no strong evidence to suggest that wild populations of amphibians are experiencing endocrine disruption. However, a recent discovery may - and only may - turn out to be one such example. The first hint of a problem in frogs came in 1993, when a group of schoolchildren discovered frogs with deformed and missing limbs in a Minnesota farm pond, and posted their finding on the Internet! In the following years, similar deformities in many different species of amphibia have been reported from many different locations throughout the USA and Canada (e.g. [32]),despite the fact that they were (apparently) not present until very recently. The incidence of these limb abnormalities was as high as 60% (though usually considerably lower) amongst some frog populations.

No consensus has yet been reached regarding the cause of the deformities; possibilities include chemical pollutants, increased UV radiation (due to ozone depletion), and parasitic infestations. Many of the sites at which deformed frogs have been found are close to agricultural fields that are intensively sprayed with pesticides and herbicides at certain times of the year. Hence, it has been suggested that one or more of these chemicals might be disrupting normal frog development. It has long been known that retinoic acid, a metabolite of vitamin A, plays an important role in limb formation during metamorphosis of amphibians. It has been shown that exogenously applied retinoic acids (or antagonists) disrupt limb formation. A recent report [33] has now suggested that products of the degradation of S-methoprene, a very widely used insect growth regulator (i. e. a pesticide) dramatically interferes with normal amphibian development, inducing limb deformities similar to those observed in wild amphibians. Further, the metabolites probably cause these effects because they are structural similar to some known teratogenic retinoic acids.Although (as the authors state clearly) their research does not prove that the reported limb deformities in many wild amphibians are caused by degradation products of an insecticide, it certainly provides a plausible explanation. If this explanation is confirmed, it would be an example of endocrine disruption in amphibians.

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