New research shows pharmaceuticals may be contributing to the global amphibian decline by altering the animals' hormonal systems and sexual development.

The problem is that these substances too often occur at biologically relevant concentrations in freshwater ecosystems, where amphibians like toads and frogs live.

Comparing the effects of the pill estrogen ethinylestradiol (EE2) in three amphibian species revealed that it could lead to complete feminization of genetic males. Until now, this has remained partly unnoticed in the wild, according to scientists from Germany's Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) and the University of Wroclaw in Poland.

"Amphibians are almost permanently exposed to such threats. Only, if we will be able to actually access these risks, we will be able to eliminate them in the long term," said Matthias Stöck, an evolutionary biologist and principal investigator of the study.

For example, 17α-Ethinylestradiol (EE2) is a synthetic estrogen commonly used in female contraceptive pills. Even though this substance does not occur naturally in the environment, it often infiltrates water bodies at alarming concentrations, as sewage treatment plants are unable to remove it completely.

Since the sensitivity towards hormonally active substances, like EE2, is not the same in all amphibians, researchers chose to focus on three different species: the African clawed frog (Xenopus laevis), the European tree frog (Hyla arborea) and the European green toad (Bufo viridis).

While the African clawed frog served as the team's model species, tadpoles of the other two had been raised in water containing different concentrations of EE2 for a period of three months. After being exposed to the hormonally active substances, researchers discovered a sex reversal ranging from 15 to 100 percent occurred in all amphibian species.

"EE2 is also part of our water supply and, together with other estrogen-like substances, it presents a serious risk not only for amphibians but also for humans. Our study shows that the clawed frog as model species is well-suited to study the effects of hormonally active substances in the environment," explained Professor Werner Kloas, co-author of the paper and an internationally renowned eco-toxicologist. "The effect established in this species, however, cannot be extrapolated to other amphibian species without caution."

Researchers were finally able to get a clear picture of the impact EE2 has on amphibians by using the latest molecular approaches combined with a phenotypic development analysis of the sexual organs.

"In addition to other threats, the feminization of populations may contribute to the extinction of amphibian species," Stöck added.

Their findings were recently published in the journal Scientific Reports.