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

The lungs vibrate more when they’re swollen, sending unique sounds to the eardrum.

A female green tree frog only needs to take a deep breath to locate her mate among the cacophony of frog croaks, groans, squeaks, and trills.
Hundreds of males from a number of species call out to prospective mates during mating season, filling ponds with their sounds.

Females face a huge challenge in focusing on worthy males amidst all of this crooning, similar to straining to understand a mate at a raucous gathering. However, researchers report in Current Biology that merely inflating the lungs of an American green tree frog (Hyla cinerea) can make her eardrums less sensitive to the sounds of other animals.

“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 simply taut tissue that vibrates as sound waves strike it, eventually converting the natural world’s bleating and buzzing into signals that are processed in the brain. Eardrums and lungs seem to mammals like us to be entirely unrelated. However, there is a direct link between the body parts of frogs that runs through the throat and into the frog’s head via an open space.

Earlier studies suggested that by having an extra input of sound, this lung-to-ear connection could help a frog pinpoint the call of a potential partner, but Lee and his colleagues at the University of Minnesota found that this theory didn’t hold true when they tested it.

Instead, when they used a laser vibrometer to test vibrations from a distance on frogs bombarded with sound waves in the lab, they discovered something much more peculiar.
Something odd occurred between 1,400 and 2,200 hertz when the researchers played a suite of sounds to females.

The bloated lungs of the frogs resonated with extra vibrations within that range, and the action of the eardrum quieted by four to six decibels on average.

“That’s a difference that would be noticeable by a frog,” Lee says. The extra noises in the lungs, for some reason, cancel out sounds of the same frequency at the eardrum, lowering sensitivity in this range.

This decrease in sensitivity occurs right between the two most influential frequencies in a male green tree frog’s croak, implying that enlarged lungs have little effect on a female’s ability to hear her own species. However, the decrease coincides with the dominant frequency of five species, such as bullfrogs and barking tree frogs, that are frequently heard calling at the same ponds.

The exact mechanism by which the lungs quiet these sounds at the eardrum is unknown, but the net result, according to the researchers, is a substantial reduction in ambient noise, allowing females to concentrate on the calls that matter.

“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.”

According to Ryan, these frogs live in incredibly noisy environments, and sifting through all of the noise to locate appropriate signals takes a lot of brain processing capacity. “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.”

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