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What Can Sexy Robot Frogs Teach Us About Evolution?

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A robotic frog inflates its vocal sac in order to “attract” female Tungara frogs. Photo courtesy Michael Ryan, University of Texas at Austin.

He’s no Kermit romancing Miss Piggy, but a robotic frog is wooing real frogs to help explain the evolution of complex behaviors like picking a mate, a new study says.

A female Túngara frog trying to choose a mate faces a complicated situation. Males gather in breeding choruses to advertise their availability, inflating their vocal sacs and producing mating calls. Much like humans in a noisy bar, female frogs need to extract meaningful information from this cacophony of sights and sounds in order to choose Mr. Right.

Biologists Michael Ryan and Ryan Taylor, at the University of Texas, Austin and Salisbury University in Maryland respectively, wanted to see how female frogs would perceive and react to mating calls that didn’t match what they would normally see and hear in nature. So they created a robotic male frog. Their results were published last month in the journal Science. (Related: “Cheetah-Cub Robot Created: See Other Nature-Inspired Machines.”)

What Female Frogs Want

Male Túngara frogs attract females with a whine-like call. They have two ways of making their whine even sexier: adding a “chuck” sound right after the whine, and inflating their vocal sacs as they call.

Ryan and Taylor played around with the timing of these visual and auditory elements. Their robotic frog produced a basic whine, but then after a pause, it either inflated its vocal sac or added on a chuck.

See a video of the robotic frog in action.

The female frogs weren’t very impressed with either the late vocal sac expansion or the late chuck. In both cases, they reacted as if the frog had just whined.

But something interesting happened when the two unattractive late cues were strung together. The vocal sac “perceptually rescued” the late chuck and bound it together with the earlier whine. The resulting signal was as attractive to the females as a normally timed whine-chuck call.

Finding Meaning Amidst Noise

Why did combining the two late signals suddenly make the call sexy? Ryan compared it to what our own brains do in “continuity illusion” tests. In these tests, a pair of beeps are played. If loud-enough white noise is played in between the beeps, people will hear the beeps as a continuous tone, rather than a pair of beeps interrupted by noise.

Scientists think the phenomenon is related to our brains’ ability to filter out background noise. “We need to be able to hook things together perceptually in unexpected ways to extract meaningful stimuli from a lot of noise,” Ryan said in a press release.

Connecting elements of the same signal together while filtering out background noise is also essential for female Túngara frogs. In crowded and confusing breeding choruses, they need to be able to tell which male is producing the most attractive call, even as multiple males are making overlapping calls and visual signals.

Ryan and Taylor think the vocal sac acted like the white noise in the continuity illusion, binding the whine and chuck together into a single, sexy signal.

Evolving Complexity

According to Ryan, the study helps explain how complex traits, like the multisensory Túngara frog mating signal, evolve.

“It’s an example of how complex traits could emerge from simpler ones,” Ryan said. The neural machinery that allows frogs to extract meaning from noisy signals, combined with the chaos of breeding choruses, could lead to the development of new ways of communicating and perceiving information over time.

The search for meaning may be a driving force in the evolution of complex behaviors. It certainly helps female Túngara frogs pick out their prince among all those frogs.