Robotic Frogs Trick Females into Believing They're More Attractive as Mates

Túngara frogs are known for their loud mating calls which are accompanied by massively inflated vocal sacs. Now, scientists have found something unusual about these calls; they've discovered that the combination of two "wrong" mating calls from a robotic frog is appealing to female frogs.

When a túngara frog seeks a mate, she will listen for the sounds that a male makes. These mating calls are based on a pattern of "whines" and "chucks." If the male is in eyesight, his inflating vocal sac will make his calls extra appealing, lending more strength to his whines. Yet more appealing still is the whine-chuck, a call that becomes irresistible when paired with the sight of his bubble-like throat.

In order to examine the visual and auditory signals in these romantic encounters, researchers decided to turn to technology. They recorded a basic whine; then they added a robotic frog that inflated its vocal sac late. They then ran a parallel experiment with a robotic frog that made a "chuck" noise that arrived late relative to the whine.

So what did the females think of these robotic frogs? On their own, it turned out that neither the late vocal sac expansion nor the sluggish chuck added any sex appeal to the fakes. In both cases it was as if the frog had just whined. But when the late cues were strung together, something unusual happened. The scientists found that the vocal sac "perpetually rescued" the chuck and bound it together with the first part of the whine-chuck call. In fact, the resulting signal was just as attractive as a well-timed whine-chuck.

"It never would happen in nature, but it's evidence of how much jury-rigging there is in evolution, that the female can be trick in this way," said Michael Ryan of the University of Texas at Austin in a news release.

So what does this tell us about the frogs? It's similar to a "continuity illusion" in humans. If loud enough white noise is played between a pair of beeps, people will begin to perceive the beeps as a continuous tone. It's possible that this is a byproduct of our brains' ability to filter out background noise. Since female frogs encounter a relatively noisy environment during breeding scenarios, it's possible that they possess neural mechanisms that allow them to correctly parse these stimuli in nature that are being "hijacked" by this artificial scenario.

"It's an example of how complex traits could emerge from simple ones," said Ryan in a news release. "In this case there's no obvious advantage to those two behaviors being hooked together in this way, but think of how you can take a muscle and move its insertion on the bone and have a great influence on speed. You didn't get the evolution for these bones and muscles all at the same time, but just making a change or adding a muscle, now you change the functional coupling. You end up with something really complex, but it evolved in a really simple way. I think in this case we may be seeing an example of how that could happen."

Want to see these frogs in action? Check out the video below, courtesy of YouTube.