Ever tried picking someone up at a loud, crowded bar? It’s not easy – not only may they not hear your fabulous pick-up line, but getting them to recognize that you’re a compatible mate with your witty follow-up with that much din in the background is a feat. If you’ve ever faced this challenge, you’re not alone – frogs feel it, too.
But, according to new research published in the journal Current Biology, frogs have a specific mechanism to navigate the complicated background noise that exists in the frog-bar tropics: their lungs. With inflated lungs, frogs can generate the effect of noise-canceling headphones, inhibiting the frequency range that their eardrums pick up on.
"In essence, the lungs cancel the eardrum's response to noise, particularly some of the noise encountered in a cacophonous breeding 'chorus,' where the males of multiple other species also call simultaneously," says Norman Lee from St. Olaf College in Minnesota, who was the lead author on the study.
This phenomenon is known as "spectral contrast enhancement," making reference to how the inflated frog lungs spotlight a male's call over other competing noises.
"This is analogous to signal-processing algorithms for spectral contrast enhancement implemented in some hearing aids and cochlear implants," says senior author Mark Bee of the University of Minnesota-Twin Cities. "In humans, these algorithms are designed to amplify or 'boost' the frequencies present in speech sounds, attenuate or 'filter out' frequencies present between those in speech sounds, or both. In frogs, the lungs appear to attenuate frequencies occurring between those present in male mating calls. We believe the physical mechanism by which this occurs is similar in principle to how noise-canceling headphones work."
The researchers were able to show by way of laser vibrometry that the resonance of inflated lungs selectively reduces the tympanum’s sensitivity to frequencies between the two spectral peaks present in the mating calls of frogs of the same species. This unexpected functional role that the lungs play in hearing suggests an evolutionary adaptation never before explored. The researchers plan to continue investigating the physical interaction between the three sources of sound in frogs (external, internal via the opposite ear, and internal via the lungs) that determine the eardrum's vibration response.