The male Bornean rock frog cannot scream over the sound of a waterfall. Instead, he threatens other frogs with his feet. The frog intimidates his male competitors with a can-can-like gesture: kicking his leg up into the air, fully extending his splayed foot, and dragging it down toward the ground.
This foot-flagging display may not sound threatening to a human, but its effect has to do with a frog’s visual perception.
To a frog, the world contains two kinds of objects: things that are worms, and things that are not worms.
If a frog sees a skinny object moving parallel to its long axis — like how a worm travels along the ground — it sees dinner. But if a frog sees a similar shape moving perpendicular to its long axis — very unlike a worm — it sees a threat to flee from. Scientists call this latter movement the anti-worm stimulus, and it strikes fear into the hearts of frogs.
Frogs likely evolved this visual system to hunt worms and stay safe from larger predators. Now, researchers suggest some male frogs have evolved to take advantage of their froggy brethren’s fears by kicking and lowering their legs in a gesture that looks a lot like an anti-worm signal, as a way to frighten their competition.
In a paper published Wednesday in Proceedings of the Royal Society B, researchers reveal that they could amplify the foot-flagging behavior of Bornean rock frogs by giving the frogs a dose of testosterone. The hormone acts on the muscles in the frog’s leg to exaggerate the gesture, meaning the more testosterone coursing through the frog, the bigger the foot-flagging display.
This flamboyant foot display, intensified by the sex hormone, suggests the frogs evolved a way to exploit their competitors’ unusual visual system to appear more dangerous to other frogs.
The new paper “provides an insightful perspective about how this hormone affects a neat visual display, foot-flagging, but also about what those changes may mean for the frogs seeing them,” Ximena Bernal, a behavioral ecologist at Purdue University who was not involved with the research, wrote in an email.
Bornean rock frogs are one of many frog species that wave their feet to communicate. In the wild, male Bornean rock frogs congregate by waterfalls and fast-flowing streams, which are very noisy. So the frogs evolved the visual signal of foot-flagging. The frogs have white webbing between their toes, making their feet even more visible among the dark rocks.
In the wild, it appears foot-flagging only has meaning among male frogs. When a female wanders to the stream, she exhibits little preference and will mate with the first male she sees. “But even while the male is on the female, he still foot flags,” said Doris Preininger, a researcher at the Vienna Zoo and author on the paper.
“Some species do it with both feet simultaneously,” said Matthew Fuxjager, a biologist at Brown University and an author on the paper.
Fuxjager had previously researched how smearing a dose of testosterone on the frogs increased the frequency of foot flagging, but he and Nigel Anderson, a graduate student in his lab and an author on the new paper, wanted to further investigate.
They dug into older studies and learned a few researchers had proposed that a frog’s worm-anti-worm worldview may have influenced the evolution of foot-flagging. But no one had looked into it.
So Fuxjager and Anderson hatched a plan to record foot-flagging frogs at the Vienna Zoo — some injected with testosterone and others with a saline placebo. They wanted to see if the hormone would affect the flagging behavior. And if it did, they wanted to know if the hormone would make the foot flag look even less like a worm (and more like a threat).
At the zoo, Anderson would inject a frog with testosterone, place it in a clear box inside a larger terrarium full of frogs, and wait, camera in hand, for the frog to flag.
On some days, six hours passed and the injected frog did not show feet. Other days, Anderson got the perfect shot: a tiny frog kicking out one of its legs and revealing its bright white toe webbing.
Anderson then watched the videos frame-by-frame and tracked each flagging frog’s big toe to calculate whether the testosterone-dosed frogs produced a bigger flag. They did, stretching their legs 10 millimeters higher than the other frogs — the height of an adult male Bornean rock frog sitting upright. The more vertical the foot flag, the more threatening the gesture is to competitors.
The researchers say the sex hormone’s influence on the exaggerated leg kick suggests the frogs evolved the intimidating gesture because it exploits their male competitor’s visual system.
“Together these things are going to create this recipe by which you get a lot of limb-shaking,” Fuxjager said.
Jenny Ouyang, a physiologist at the University of Nevada, Reno, who was not involved with the research, said a future experiment could test the reactions of male frogs to a testosterone-induced flagging display, to see if the observing frogs do perceive the testosterone-enhanced flag as more threatening.
But the rock frogs’ eccentricities, and the difficulty of filming them, complicate such a test. The dosed frogs only reliably flag while surrounded by a group of other frogs, which are all tiny and indistinguishable from one another.
Ouyang jokingly suggested a workaround: equipping the frogs with virtual reality goggles. “But I’m not sure they make goggles that small,” she said.