Frogs have been trying to mate with odd things for 220 million years

Male frogs will sometimes try to mate with turtles or inanimate objects, and now there is evidence that the behaviour began deep in prehistory with the first frogs.

Mating frogs may have been occasionally getting it wrong for hundreds of millions of years. We know that males today will sometimes select an inappropriate partner during the breeding season – a frog from a different species, a turtle, a fish or even an inanimate object. Now there is evidence that these mistaken attachments could be an ancient feature of frog reproduction, arising early in the amphibians’ evolution.

Amplexus between a male common toad (Bufo bufo) and a female European green toad (Bufotes viridis)
Jean-Pierre Vacher

Frog mating is often hard to miss. In most species it involves a process called amplexus, in which males grip onto a female tightly for hours or days at a time until the eggs are fertilised. But there are plenty of records of male frogs grappling an unpromising target such as a frog from a different species or a dead individual. One explanation is that such mistakes are more likely to happen in species that breed in large numbers with a low ratio of females to males, and where multiple species occupy the same breeding pond.

“For a male facing huge competition with rivals to reproduce, it would be advantageous for males to arbitrarily be attracted to – and thus clasp – any female-looking object in order to increase mating probabilities,” says François Brischoux at La Rochelle University in France.

But recent data shows that misdirected amplexus is also widespread in frogs and toads that don’t reproduce like this. If such mistakes are ubiquitous in the amphibians, it is possible that misdirected amplexus emerged at the dawn of frog evolution, says Brischoux.

Two western toads (Anaxyrus boreas) in amplexus
Wirestock Creators/Shutterstock

To test this idea, Brischoux and his colleague Léa Lorrain-Soligon used a database containing 280 records of misdirected amplexus across 159 frog species. The researchers plotted these records across a frog evolutionary tree and then used a probability analysis to calculate when, in the evolutionary history of the group, the behaviour most likely first appeared.

They found a very high likelihood – over 97 per cent – that the earliest frog species made these kinds of mistaken matings. This animal is estimated to have lived about 220 million years ago.

“Our results suggest that the reproductive strategies promoting misdirected amplexus were possibly occurring early during [frog and toad] diversification,” says Brischoux.

“Frogs are an ancient group of vertebrates, with fossils found even on Antarctica. Studies like this further support the idea that even the most evolutionarily divergent frog lineages share highly conserved behaviours,” says Aryeh Miller at Washington University in St. Louis in Missouri.

Filipe Serrano at the University of São Paulo in Brazil says it is possible amphibians were occasionally getting it wrong well before the very earliest frog ancestors evolved.

“The ancestor of all amphibians, including salamanders and caecilians, could have already been a misdirected amplectant,” says Serrano. “But so far reports of misdirected amplexus by salamanders are lacking.”

Whatever the precise evolutionary origins of misdirected amplexus, the number of references to the behaviour in the scientific literature has grown exponentially since the start of the 21st century, says Brischoux. A recent rise in habitat modifications, droughts and human-caused environmental noise may be partially responsible.

“The possibility that anthropogenic activities may promote the occurrence of this reproductive dead end is worrying at a time when amphibian populations are suffering a strong decline,” adds Brischoux.

Journal reference:

Biological Journal of the Linnean SocietyDOI: 10.1093/biolinnean/blad108

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