Irish scientists solve mystery around ancient frog sex death trap

8 Jul 2022

Image: © Thierry Maffeis/

Two UCC scientists have found the only possible reason hundreds of frogs died in a swamp in Germany 45m years ago – they were having sex.

Scientists at University College Cork (UCC) have just solved an ancient mystery around the cause behind the deaths of hundreds of frogs around 45m years ago, whose fossils were found intact in a pre-historic swamp in present-day Germany.

The former coalfield of Geiseltal in Saxony-Anhalt, Germany has long been a breeding ground for debate among palaeontologists on the specifics of what exactly caused the deaths of the 50,000-odd fossilised animals found in the valley, including birds, horses, bats, fish and frogs.

During a period known as the middle Eocene, nearly 50m years ago, the Earth was a much warmer place. It was then that the swampy subtropical forest in Geiseltal was home to a wide range of animals – from ancestors of the horse, large crocodiles and snakes to ground-dwelling birds, lizards and plenty of anurans, which are frogs and toads.

Of these animals, it is the mysterious circumstances surrounding the death of the hundreds of healthy forebears of modern frogs that has baffled scientists the most.

Previous studies have suggested that the Geiseltal frogs died during the desiccation of lakes, which may or may not have been accompanied by oxygen depletion in the water. But none of the studies have been conclusive, until now.

Published in the Papers in Palaeontology journal this week, the study was authored by UCC researchers Daniel Falk and Maria McNamara as well as German researcher Oliver Wings. McNamara spoke to in March of this year about how fossil records can unlock the secrets of animal evolution.

It is part of a research cooperation between UCC and Martin-Luther-University Halle-Wittenberg with funding from the Irish Research Council.

Frogs ‘died during mating’

By studying the bones of the fossil frogs, the team was able to narrow down the options to one single cause. As it turns out, the frogs died while having sex.

“As far as we can tell, the fossil frogs were healthy when they died, and the bones don’t show any signs of predators or scavengers – there’s also no evidence that they were washed in during floods or died because the swamp dried up.” said Falk, who led the study.

According to the study, most of the Geiseltal fossil frogs are species that spend their lives on land, returning to the water only to breed. “By process of elimination, the only explanation that makes sense is that they died during mating.”

Old habits die hard, and not much has changed in 45m years. The study noted that the phenomenon of death during sex is common even in frogs today.

“Female frogs are at higher risk of drowning as they are often submerged by one or more males – this often happens in species that engage in mating congregations during the short explosive breeding season,” said McNamara.

“What’s really interesting is that fossil frogs from other sites also show these features, suggesting that the mating behaviours of modern frogs are really quite ancient and have been in place for at least 45m years.”

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Vish Gain is a journalist with Silicon Republic