Frog's Lungs Act Like Noise-Canceling Headphones

Inflated lungs help frogs cancel out extraneous noise, allowing them to zero in on the calls of potential mates. They balloon up, essentially acting like noise-canceling headphones, researchers report in a new study.

Think of it as the pre-pandemic cocktail party problem. Everyone is chatting all around you in a crowded room, making it nearly impossible to actually hone in on a conversation from someone you want to listen to.

Vocal signals are the primary way that males attract females in most of the 7,200-plus species of frogs, points out study senior author Mark Bee of the University of Minnesota-Twin Cities. 

Imagine a single crowded pond where so many frogs are calling at once, struggling to be heard over other noise, including the sounds of other frog species.

“Listening frogs possess a number of mechanisms that help them pick out calling males in noisy situations,” Bee tells Treehugger.

“These include things like taking advantage of spatial separation between calling individuals or between calling individuals and the direction of dominant sources of noise.”

Frogs also take advantage of brief "dips" in the level of background noise to catch what Bee refers to as "acoustic glimpses" of calls of interest. They also take advantage of the natural differences in frequency between species, and maybe between individual frogs too.

But the frog’s inflated lungs play a key role. They lower the eardrum’s sensitivity to environmental noise in a specific frequency range, the researchers found. That improves how well females hear mating calls of males in the same species.

"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 lead author Norman Lee of St. Olaf College in Minnesota.

The results were published in the journal Current Biology.

Cancelling the Eardrum's Response

The researchers explain that what the lungs are doing is called “spectral contrast enhancement.” It makes the male’s mating call stand out in relation to other noise at adjacent frequencies.

That’s comparable in some ways to signal-processing algorithms used in some hearing aids and cochlear implants, Bee says.

“In humans, these algorithms are designed to amplify or ‘boost’ the frequencies present in speech sounds (i.e., the signal), attenuate or ‘filter out’ frequencies present between those in speech sounds (i.e., the noise), or both. In frogs, the lungs appear to attenuate frequencies occurring between those present in male mating calls,” he says.

“We believe the physical mechanism by which this occurs is similar in principle to how noise-cancelling headphones work," Bee explains.

For their study, the researchers used data from a citizen science project called the North American Amphibian Monitoring Program. The 15 years of data allowed them to figure out which frog species were most likely to “co-call” with the species they were studying, the green treefrog. 

They found that 42 different species co-call along with green treefrogs, but just 10 of those species account for nearly 80% of observed reports of co-calling. They used a combination of their own recording of frogs and other curated recordings to analyze calls of those 10 species.

Their analysis suggests that the green treefrog’s inflated lungs would make it more difficult to hear calls of other species while leaving their ability to hear the calls of their own species.

“Needless to say, we think this result — a frog’s lungs canceling the eardrum’s response to noise created by other species of frogs — is pretty cool!” Bee says.