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A male Borneo rock frog cannot scream at the sound of a waterfall. Instead, it threatens other frogs with its feet. The frog scares his male opponents with a tin can-like move: he raises his leg in the air, fully extends his open foot, and drags it towards the ground.
This display of foot markings may not sound threatening to a human, but its effect has to do with a frog’s visual perception.
For a frog, the world contains two kinds of objects: things that are worms and things that aren’t.
If a frog sees a skinny object moving parallel to its long axis – like how a worm moves on the ground – it sees its supper. But if a frog sees a similar shape moving perpendicular to its long axis – very different from a worm – it sees a threat to escape. Scientists call this second action an anti-worm stimulant, and it strikes fear into frogs’ hearts.
Frogs likely evolved this visual system to hunt worms and dodge larger predators. Now, researchers suggest that some male frogs have evolved to take advantage of their tadpole brethren’s fears by kicking and lowering their legs in a move much like an anti-worm signal, as a way to intimidate their competition.
In an article published Wednesday Proceedings of the Royal Society B, researchers have revealed that by giving the frogs a dose of testosterone, they can strengthen the foot-tapping behavior of Bornean rock frogs. The hormone acts on the muscles in the frog’s leg to exaggerate movement, meaning the more testosterone flowing through the frog, the larger the foot marking display.
This showy foot image, intensified by the sex hormone, shows that frogs have evolved a way to exploit their competitors’ unusual visual systems to appear more dangerous to other frogs.
The new paper “provides a neat visual display of this hormone, insightful insight into how it affects foot marking, but also about what these changes might mean for frogs,” says behavioral ecologist Ximena Bernal of Purdue University. Wrote an email with research.
Borneo rock frogs are one of many frog species that wave their feet to communicate. In the wild, male Borneo rock frogs are collected by waterfalls and fast-flowing streams, which are very loud. Thus, frogs developed the visual signal of foot marking. Frogs have a white web between their toes, which makes their feet even more visible among the dark rocks.
In the wild, marking with feet seems to have meaning only among male frogs. When a female wanders the river she shows little preference and will mate with the first male she sees. “But even when the male is on the female, he still puts the flags on his feet,” said Doris Preininger, a researcher at the Vienna Zoo and author of the paper.
“Some species do this with both feet at the same time,” said Matthew Fuxjager, a Brown University biologist and author of the paper.
Dr. fuxjager before researched How giving frogs a dose of testosterone increases the frequency of foot bounce, but he and Nigel Anderson, a graduate student in his lab and author of the new paper, wanted to investigate further.
They searched through earlier studies and learned that several researchers had suggested that a frog’s worm-anti-worm worldview may have influenced the evolution of foot marking. But no one had looked at it.
That’s why Dr. Fuxjager and Mr. Anderson devised a plan to record stamping frogs at the Vienna Zoo – some injected with testosterone and some with a saline placebo. They wanted to see if the hormone could affect marking behavior. And if so, they wanted to know if the hormone would make the foot flag look less worm-like (and more like a threat).
At the zoo, Mr. Anderson would inject a frog with testosterone, put it in a clear box inside a larger terrarium filled with frogs, and wait for the frog to be marked with the camera in hand.
On some days, six hours passed and the injected frog did not show its feet. On other days, Mr. Anderson had the perfect shot: a small frog kicking one of his legs and revealing a bright white toe plait.
Mr. Anderson then watched the videos frame-by-frame and tracked the big toe of each marked frog to calculate whether the testosterone-dose frogs produced a larger flag. They did so by extending their legs 10 millimeters higher than other frogs—the height of an upright sitting adult male Bornean rock frog. The more vertical the foot flag, the more threatening the gesture to opponents.
The researchers say the sex hormone’s effect on the exaggerated leg kick suggests that the frogs develop a frightening gesture as they abuse the visual system of their male rivals.
Dr. “Together, these things will make up this recipe that causes you to shake a lot of limbs,” Fuxjager said.
A future experiment could test the responses of male frogs, said physiologist Jenny Ouyang of the University of Nevada, Reno, who was not involved in the research. – Improved flag to be more menacing.
But the quirks of rock frogs and the difficulty of filming them complicate such a test. Dosed frogs are reliably marked only when surrounded by a group of other frogs, all of which are small and indistinguishable from one another.
Dr. Ouyang jokingly suggested a workaround: equipping frogs with virtual reality glasses. “But I’m not sure they make glasses that small,” he said.
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