Why Do Frogs Keep Trying to Mate with the Wrong Things?

A shoe, a coconut, a tennis ball, a dead frog, a turtle, a mango, a ruler, a gecko, and a lump of yack dung — these are just some of the odd things frogs have been recorded trying to have sex with.

Frog sex, in most species, involves the male gripping onto the female from behind for long periods of time — from hours to days — until they’ve succeeded in fertilizing their eggs. But mating can be very competitive for these web-footed amphibians, resulting in some individual mistakenly trying to copulate with things that can’t actually give them any offspring, a phenomenon known to science as “misdirected amplexus”.

This strange behavior has long puzzled scientists. They're continuing to dig into its origins, potential benefits and the reasons behind it, as well as whether climate change is to blame for it seemingly becoming more and more common.

In the 100 years between 1920 and 2020, scientists have formally taken note of at least 378 cases of “misdirected amplexus”, according to a 2022 research paper published in the journal Ecology.

Although such cases are mostly accidental flukes, this behavior seems quite widely distributed: It's present in almost all branches of the frog tree of life, as the records come from 156 species from 18 different families and 52 countries from all continents, except the one with no frogs, Antarctica.

“It’s actually very widespread in every scale that you look into,” says study author Felipe Serrano, an ecologist at the University of São Paulo, in Brazil. “You can find it in forests, savannas, temperate regions, and tropical regions.”

In most cases — 282 to be precise — these male frogs were attempting to mate with frogs from other incompatible species that they would not be able to reproduce with. Fair mistake. But the records show that in 46 of the 378 cases, the misdirected amplexus was carried out on a dead frog; meanwhile, there were 50 recorded times where it was carried out on other animals or objects.

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The primary theory experts use to justify this behavior is simple: as a strategy to overcome competition. Mating only occurs during specific times of the year, and female frogs are often largely outnumbered by their male counterparts, so the rivalry is born of necessity — better to be overzealous than sleep on the prize.

“Grasp, ask questions later,” says Serrano. “There's a positive outcome of this behavior as a whole.” And sometimes, the act of misdirected amplexus actually can yield offspring, as there have been cases of dead female frogs carrying eggs that could still be successfully fertilized.

Still, misdirected amplexus isn't always (or even mostly) beneficial for frog reproduction. That's because every second spent on a mate that isn’t actually going to be bearing you any children — because, for example, she happens to be a rock — is time wasted in the precious game of mating, meaning that a male’s precious chance at reproduction could have been wasted until next year.

“So for the individual who spent two hours attempting to mate with a rock, there's no upside whatsoever. But as a whole, if you look at populations, the ability to grab the first thing that you see is actually a very good trait to have,” says Serrano, who explains that the evolutionary system favors individuals that have honed their ability to quickly find a good mate.

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Building off records like those in the 2022 study, scholars from France looked into the roots of this odd behavior of frog mating. The team, led by François Brischoux, an amphibian researcher from the French National Centre for Scientific Research, used data from 280 cases of misdirected amplexus, tracing the frogs’ family trees and evolutionary histories.

Researchers used probability to figure out when this behavior could have first emerged.

“Knowing the current characters of this extant species, we can go back in time and calculate the probability that these characters occurred earlier,” says Brischoux. “It's a statistical process; you just obtain the probability that, at each node backward in the time of genetic tree, a given character — it could be colors, or behavior, or mating strategy — occurred.”

According to their calculations, published in the Biological Journal of the Linnean Society in 2023, there’s more than a 97% chance that this behavior dates back to antiquity. In other words, even the first ancestors of the modern frog, who lived a whopping 220 million years ago, were likely already trying to mate with everything in sight.

“It’s a bit fuzzy; we don't know really when such behavior first occurred," says Brischoux. "But what we found is that the probability that these behaviors were occurring very early before the diversification of this group is very likely."

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Its ancient origins aside, the number of times misdirected amplexus has been recorded in frogs in the past hundred years seems to be growing exponentially over time — especially since the start of the 21st century.

This could be a consequence of the fact that researchers only recently started taking note of this behavior in published literature. In other words, as of now, there isn't enough data to know whether there’s been an increase in sexy mistakes or just an increase in reports of it. Scientists would have to start to closely monitor a bunch of frog populations over time and see whether this behavior increases or decreases in the coming years in order to be sure.

“Maybe studying very well-known species in well-known areas will help us to understand if it's really happening more often, but currently, we just don't have this answer,” says Brischoux.

Still, both Brischoux and Serrano suspect that increased human interference with nature —and the fragmentation of natural habitats and biodiversity — might play a role in frogs getting extra frisky recently. And frogs are much more likely to come across objects like boots, rulers, plastic cups, and tennis balls, as humans have been increasingly invading their turf.

In short: “The range of things they can accidentally grasp, is a lot more than what there used to be 100 or 200 years ago,” says Serrano, who is currently working to pinpoint more details about misdirected amplexus — and whether rising global temperatures, in particular, might play a role.

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