Female green tree frogs have noise-canceling lungs that help them hear mates

To find her mate amidst a cacophony of frog croaks, groans, squeaks and trills, a female green tree frog just needs to take a deep breath.

During mating season, ponds resound with the sounds of hundreds of males from many different species crying out to potential mates. Homing in on eligible males against all this crooning presents a significant challenge for females, akin to straining to understand a friend at a raucous party. But by simply inflating her lungs, an American green tree frog (Hyla cinerea) can make her eardrums less sensitive to the sounds of other species, researchers report March 4 in Current Biology. 

“We think the lungs are working a bit like some noise-canceling headphones,” says Norman Lee, a neuroethologist at St. Olaf College in Northfield, Minn., allowing females to filter out environmental noise at the eardrum itself.

An eardrum is just taut tissue that vibrates when sound waves hit it, ultimately translating the bleating and buzzing of the natural world into signals that get processed in the brain. To mammals like us, eardrums and lungs seem completely unrelated. But there’s a direct connection, via an open space, between the body parts in frogs that runs through the throat and into the frogs’ head. That lets frog eardrums pick up sound from outside the ear and also register vibrations from the lungs.

Earlier research hinted that this lung-to-ear connection might boost a frog’s ability to pinpoint the call of a potential mate by providing an extra input of sound, but that hypothesis didn’t pan out when Lee, who conducted the research at the University of Minnesota, and his colleagues tested it. Instead, they found something even more unusual when they aimed a laser vibrometer, which measures vibrations from a distance, at frogs bombarded with sound waves in the lab.

When the researchers played females a suite of sounds, something strange happened between 1,400 and 2,200 hertz. Within that range, the frogs’ inflated lungs resonated with extra vibrations and the movement of the eardrum quieted, by the equivalent of four to six decibels, on average.

“That’s a difference that would be noticeable by a frog,” Lee says. Somehow, the extra vibrations of the lungs cancel out sounds of the same frequency at the eardrum, reducing sensitivity in this range.

This dip in sensitivity falls just between the two most prominent frequencies of a male green tree frog’s croak, suggesting that inflated lungs don’t affect a female’s ability to hear her own species. But the dip does coincide with the dominant frequency of five species that are often found calling at the same ponds, such as bullfrogs and barking tree frogs. How precisely the lungs quiet these sounds at the eardrum remains unclear, but the net effect is a significant reduction in environmental noise that allows females to focus on the calls that matter, the researchers say.

“I was almost overwhelmed by this paper,” says Mike Ryan, an evolutionary biologist at the University of Texas at Austin. “It shows that the function of the eardrum isn’t static, but can be dynamically changed by the lungs in a way that reduces sensitivity to frequencies that aren’t important to the frog.”

These frogs are in really noisy environments, Ryan says, and sifting through all that noise to find relevant signals requires a lot of processing power by the brain. “This lung trick really cleans up the sounds” before they even reach the brain, Ryan says. “We don’t think of the lungs playing a role in hearing, but the way this is working is just really, really cool.”