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Female Frogs Can Selectively Mute Unwanted Suitors

Female frogs have the uncanny ability to block out unwanted sounds, which comes especially handy during the ribbeting spring mating season.

(CN) — Female frogs have the uncanny ability to block out unwanted sounds, which comes especially handy during the ribbeting spring mating season.

Around April frog habitats become a chorus of mating calls, so much so that picking out an individual of the same species, let alone one in whom a female frog is interested, becomes quite a challenge. Like a big cocktail party, a cacophony of noise muddles together everyone’s favorite pickup lines.

Researchers were able to determine nature’s solution to this problem, for frogs anyway, and published their findings in a new study Thursday in the journal Current Biology.

Female frogs can tune their ears to a specific sound range by inflating their lungs, thereby canceling out mating calls from other species or simply from other frogs they don’t fancy. When their lungs inflate, the eardrum’s sensitivity is reduced in a certain frequency range, thus improving the signal to noise ratio in the frequencies they’re focused on.

This is similar to how noise-canceling headphones and hearing aids work; rather than turning up the volume in a noisy environment, an algorithm accounts for unwanted noises and effectively mutes them. And like many technological breakthroughs, nature got there first.

"'Spectral contrast enhancement’ is a signal processing strategy that can be applied to better hear signals in noise,” explained lead author Norman Lee of St. Olaf College in Minnesota, in an email. “Engineers have been developing such signal processing techniques to improve the function of hearing aids and cochlear implants for decades, which precedes our discovery that female frogs can use this same approach to improve their ability to hear conspecific advertisement calls in noise.”

This lung to ear sound transmission pathway in frogs has been known for more than 30 years. Previous research suggested that it was a means of directional hearing, meaning it was once thought that female frogs could zero in on the mating calls they wanted to hear by listening only in a certain direction.

By measuring the vibration of a frog’s ear in response to sounds from multiple sources using a technique called laser Dopple vibrometry, the authors determined that wasn’t the case and set out looking for another explanation. They noticed that female frogs can hear males of their own species regardless of whether or not their lungs are inflated, so that didn’t have an impact on volume or directionality.

“We expected the directionality of the two ears (i.e., how much their relative vibrations changed with changes in sound location) to be much greater when the lungs were inflated,” said the authors in a related Q&A. “But they weren’t, not even a little bit!  This surprising result allowed us to reject a longstanding hypothesis about how the lung-to-ear sound transmission pathway might benefit the animal.”

The team now believes frogs’ lungs function similar to the microphones integrated into noise-cancelling headphones. The inflated lungs pick up sounds in a desired frequency range, then feed that data through the mouth, Eustachian tubes and middle ears, where it interacts with outside noise.

They hypothesize that the sound waves brought in by the lungs may get inverted to distinguish them from outside noises, allowing a female frog to hone in on her own species. However, the exact mechanism by which this interaction works remains a mystery, and will be the subject of further research.

The authors of this study specifically looked at American green tree frogs, but they believe their findings can likely be generalized to other frog species as well.

“One of the main takeaways for a general audience is that evolution has shaped the frog’s auditory system to potentially use spectral contrast enhancement as a strategy for hearing conspecific calls in noise, at least 200 million years before humans were able to apply it to technology,” said Dr. Lee in an email. “So the lesson here is that taking a look at how evolution has solved common problems (like hearing signals of interest in noise), might inform engineers in developing technology.”

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