Dr Hoffmann, why do hormones make it difficult for frogs to flirt?

This article was published in our newsletter. Sign up here.

Dr Frauke Hoffmann

“Relatively high concentrations of hormonally active substances can be found in waterways, particularly those in the Western world. These are usually manmade substances such as medicines or biocidal agents. They can affect the hormone metabolism of animals or indeed humans. Oestrogens such as those to be found in contraceptive pills play a particularly important role in this context. When they are excreted from the body in the urine, they still retain all their functional properties. Sewage treatment plants are usually unable to filter such substances completely out of the water, with the result that they find their way into waterways where they accumulate.

From sewage treatment plant to river

Fish and frogs are exposed to these substances in the waterways and absorb them into their bodies. Because the substances imitate the effects of natural female sex hormones, they disrupt the hormonal balance of the animals. I study this effect in South African clawed frogs. They are ideal research specimens not only because their biology and physiology are already well-researched, but also because they live entirely in water.

Changes in frog calls


In my tests I noted that male frogs react with great sensitivity to oestrogenic substances. They engaged in less courting of their mating partners and also changed the emphasis of their calls. The call parameters changed so significantly that females found the calls less attractive. If given the choice, females would give preference to non-contaminated males. And even where no other choice was available, the changed calls of the males resulted in the females being less willing to mate. In the longer term, this could lead to less successful mating among these frogs, and possibly even to entire frog populations in hormone-polluted waterways becoming extinct.”

Dr Frauke Hoffmann is a research associate in the Ecophysiology and Aquaculture department of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin. Within the framework of the “XENOCALL” project, she is researching current contamination levels in selected surface waters and their impact on the clawed frog.

More information

Leibniz-Institute of Freshwater Ecology and Inland Fisheries: