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For the last two decades, strange things have been happening to frogs. Some frog populations have high rates of limb deformities, while others have high incidences of what is known as “intersex” – traits associated with both males and females, such as male frogs whose testes contain eggs.

David K. Skelly, professor of ecology at the Yale School of Forestry & Environmental Studies, set out to discover what was causing these deformities, which some researchers were attributing to the use of an agricultural pesticide called atrazine. Skelly launched an experiment in ponds throughout Connecticut, studying frogs in four landscapes: forests, agricultural areas, suburbs, and cities. And what he found was surprising – the highest rates of deformities were not occurring in and around farmlands, but in cities and suburbs.

David Skelly In an interview with Yale Environment 360 contributing writer Carl Zimmer, Skelly described what chemicals may be causing these abnormalities in frog populations, and explained why this phenomena may have troubling implications not only for amphibians, but for other vertebrates, including humans. One thing seems clear: The deformities showing up in frogs are almost certainly not caused by a single chemical, but rather by a whole suite of substances – including medicines excreted by humans into the environment – that act in concert to mimic hormones like estrogen or cause other ill effects.

“The fact that these kinds of estrogens out in the environment can have this kind of effect on a vertebrate – many people would say that that alone is a basis for us to be concerned,” says Skelly.