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Female red-eyed tree frogs are more than twice as likely to select a mate if they can feel the vibrations of their call instead of only hearing it.

These small, bright-green frogs (Agalychnis callidryas) are about the size of a chicken egg and are found in rainforests from southern Mexico to northern Colombia. When the males want to woo a female or ward off a rival, they use air sacs in their throat to send a chorus of croaks and chirps.

When they call out, they also send vibrations through the plant limb they are on. Until now, no one had looked at what role those vibrations might play in mate selection.

“We think we understand how sound communication works… but we’re missing part of the picture,” says at Gettysburg College in Pennsylvania. “It kind of blows my mind that we haven’t been looking at this already.”

He and his colleagues designed an experiment to examine how frogs reacted to calls they could hear compared with “combination calls”, those they could both hear and feel.

First, they tested wild male frogs’ reactions in four different scenarios: a call, a vibration, a combination call and no call or vibration. Caldwell outfitted a sapling in the species’ native range in Panama with speakers and a subtle mechanical shaker to mimic the presence of a rival frog. The team tested each animal’s response to 283 calls from 63 unique males.

As Caldwell suspected, males had more frequent and varied displays of aggression – body shaking and push-up-like movements – in response to calls that they could both hear and feel. “When I played the sound plus vibrations, the males freaked out,” says Caldwell.

The researchers repeated a version of the experiment with females, this time on a Y-shaped branch that spanned about 2 metres. The top of each limb of the branch had a speaker playing a recording of a mating call, and one side also included a shaking device. They alternated playing just a call and playing a combination call to see which direction the female would climb.

Around 68 per cent of the females opted to travel towards calls they could feel and hear.

Caldwell thinks the frogs are picking up on vibrations to select mates already perched on the same plant. That way, females can avoid traversing dense foliage to reach to male suitor.

at Boston University in Massachusetts, who wasn’t involved in the work, suspects other tree frog species may be factoring vibrations into their rivalry and mating choices, too.

“How many other frogs are doing this, and we have been completely clueless?” says Warkentin. “Now, of course, people need to go and ask that question.”

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