For frogs, love is noisy. Each spring, swamps, marshes and ponds across the United States become the amphibian equivalent of raucous singles bars as a host of damp-skinned hopefuls from many species cry out, seemingly all at once, in hopes of attracting a mate.
Males of each species have their own songs to sing, and, somehow, females have to sift through the noise to pick out not just the calls of their own kind but also the individual voice of some male fit to fertilize her eggs. Across the green tree frog’s range in the southeastern U.S., more than 40 other species are known to belt out their respective serenades while the greens are trying to partner up. For this lime-green, roughly two-inch frog, trying to find a mate is such an ear-ringing affair that, according to research published today, it uses its own lungs as a type of noise-cancelling headphones to better hear the calls of its own species.
The new paper, published in the journal Current Biology, finds that green tree frogs pump their lungs full of air to help them solve what biologists term “the cocktail party problem.” The precise mechanism isn’t yet fully understood, but when the green tree frog’s lungs are inflated they reduce its eardrums’ sensitivity to the calls of other species without distorting or muting its own species’ calls. The findings also help explain a mysterious sonic pathway between the lungs and middle ears of most frogs that has puzzled scientists since its discovery in 1988.
Frog ears aren’t much like ours. Most frogs have their eardrums on the surface of their skin—which combative frogs sometimes exploit by trying to damage a rival’s eardrum during fights. Another twist is that frog ears are internally connected to each other and with the lungs via air-filled passages inside the mouth.
These big, open passages inside the mouth cavity, namely the glottis and the Eustachian tubes, allow sound to pass through the animal and reach the eardrums from the inside as well as the more traditional external route.
In the more than 30 years since Peter Narins, a biologist at the University of California, Los Angeles, and his colleagues discovered that the inflated lungs of most frogs conduct and transmit sounds to the middle ear, nobody had been able to pin down what, if anything, the pathway contributed to frogs’ hearing.
“We first set off trying to investigate the idea that the connection between the lungs and middle ear might somehow improve the frog’s ability to determine location of calls from their own species, which has been the main hypothesis up until now,” says Norman Lee, a biologist at St. Olaf College and lead author of the new study.
Lee and collaborator Mark Bee, a biologist at the University of Minnesota and the paper’s senior author, put this idea to the test in experiments using green tree frogs. “We found that the lungs didn’t do anything for directional hearing,” says Bee of the results the team published in the Journal of Experimental Biology in fall 2020. “It was a big, long paper of no results, but it set us up to try to say, ‘OK, what are the lungs doing then?’”
To find out, the researchers used a technique called laser Doppler vibrometry, which employs lasers and tiny reflective balls to measure the vibrations created when objects conduct sound. In the lab, the laser vibrometer showed that the female green tree frog’s lungs only resonated in response to sounds at frequencies between 1400 to 2200 hertz. When the lungs resonated, the researchers observed that the eardrums became less responsive to sounds in that frequency range.
Next, Lee and his co-authors wanted to explore if there was anything biologically significant about the frequency range that the inflated lungs appeared to dampen. By playing recordings of the green tree frog’s own calls they confirmed that a female’s inflated lungs had no impact on her sensitivity to the song of her own species. In fact, the frequency range muted by the inflated lungs nestles precisely between the two main frequency components of the green tree frog’s call.
Having confirmed that the lungs don’t stop the females from hearing the males of their own species, the team turned to the many other species at the froggy cocktail party. Using a massive database of frog calls recorded by citizen scientists in locations around the United States, the team created a list of 42 other frog species known to call out at the same times and locations that green tree frogs do. The researchers narrowed this list down to ten species that accounted for nearly 80 percent of the reported instances of co-calling and analyzed the frequencies occupied by their respective calls.
The team found that the calls of five of those species, including the two that co-called with green tree frogs most often in the citizen science database, fell directly in the frequency range that the green tree frog’s inflated lungs tuned out. By inflating her lungs, a female green tree frog can turn down the volume of the noise contributed by other species, and make it easier to hear the males of her own kind.
“So, the inflated lungs are attenuating that extraneous noise, which reduces likelihood that the neurons will respond to the wrong thing,” says Bee.
Ximena Bernal, a biologist at Purdue University who was not involved in the paper, says the study is very elegant. She likes “how they integrate the laser vibrometry to see how the eardrum responds and then bring it back to the ecology of the species to see which other types of frogs green tree frogs are calling with.”
In future research, Lee and Bee hope to explore whether these findings hold for other species besides the green tree frog. According to other researchers, it seems likely that this physiological mechanism could be widespread.
“I expect these results to extend to most species of frogs,” says Andrea Simmons, a neuroscientist who specializes in animal communication at Brown University who was not involved in the research. “We know a lot about green tree frogs and there’s nothing unusual about them in frog terms, so I would be shocked if these findings didn’t carry over to other species.”
Simmons also adds that she hopes follow-up studies probe whether the male green tree frog’s lungs work the same way, because this study only examined female frogs. For example, she says in bullfrogs the eardrums of each sex are different sizes, and in coqui frogs the hearing of males and females is tuned to focus on different frequencies.
Bernal is curious to see how this sound-cancelling mechanism might work in tropical frog species that have to contend with an even greater diversity of other noisy creatures, from frogs to birds to insects.
The catch to these revelations is that the researchers aren’t exactly sure how the resonating lungs cancel out the songs of other frog species. They suspect that what’s going on is something akin to what noise cancelling headphones do. In this scenario, the sound waves transmitted via the frog’s lungs are somehow creating what’s called destructive interference when they encounter the sound waves coming through the external side of the eardrum, cancelling out both vibrations.
For now, Bee says he doesn’t see new-fangled headphones or frog-inspired technological breakthroughs coming out of this work. “To me,” he says, “it’s just amazing to think that noise cancellation, a technology human engineers developed not too long ago, was probably first exploited by amphibian evolution 200 million years ago.”