Worldwide, up to a third of frog species are threatened with extinction. The causes can be diverse, from clime change, the attack of the fungus Batrachochytrium dendrobatidis, introduction of invasive species (including of other frogs, like the American bull frog) to contamination, which acts from directly killing them up to the appearance of malformations in frogs.

Now, a Swedish team at Uppsala University discovered a surprising way in which contamination can affect frog populations. Estrogen-like pollutants were found in lab experiments to turn male tadpoles in adult frog females.

The two tested species of frog, the northern leopard frog (Rana pipiens) from US and Canada and the European common frog (Rana temporaria) were exposed to the same estrogen amounts found in natural waters in Europe, the United States and Canada. In the control groups, the females represented under 50 % of the total number of adults, a normal ratio, but the sex ratio in frog groups raised in water with various estrogen levels was very different.

Even the lowest estrogen dose doubled the number of females. The highest estrogen levels produced 95 % female in one species, and 100 % in the other. "The results are quite alarming," said co-author Cecilia Berg, a research in environmental toxicology. "We see these dramatic changes by exposing the frogs to a single substance. In nature there could be lots of other compounds acting together."

"Earlier studies in the United States linked a similar sex-reversal of Rana pipiens male frogs -- one of the two species used in the experiment -- in the wild to a pesticide that produced estrogen-like compounds. Pesticides and other industrial chemicals have the ability to act like estrogen in the body," Berg said. "Some of sex-altered males became fully functioning females, but other had ovaries but no oviducts, making them sterile," Berg explained.

"The study does not measure the potential impact of pollutant-driven sex change for frog species, but the implications are disquieting. Obviously if all the frogs become female it could have a detrimental effect on the population," she said. "The only immediate remedy would be to improve sewage treatment in areas where frogs and other amphibians might be affected to filter out estrogen concentrations coming from contraceptive pills and from industrial pollutants."