To find her mate amid a cacophony of frog snores, moans, squeaks and trills, a female green tree frog just needs to take a deep breath.
During the mating season, the gaps resonate with the sounds of hundreds of males of many different species clamoring for potential mates. Finding eligible men against all this cantarón poses a significant challenge for females, similar to striving to understand a friend at a carnivorous party. But simply by inflating the lungs, an American green tree frog (Hyla cinerea) can make its eardrums less sensitive to the sounds of other species, researchers report on March 4 in Current Biology.
“We think the lungs work a bit like some headphones that eliminate noise,” says Norman Lee, a St. Neuroethologist. Olaf College in Northfield, Minnesota, which allows females to filter ambient noise into the eardrum itself.
An eardrum is just a tense tissue that vibrates when sound waves strike it, ultimately translating the babbling and buzzing of the natural world into signals that are processed in the brain. To mammals like us, the eardrums and lungs seem completely unrelated. But there is a direct connection, through an open space, between the parts of the body in the frogs that cross the throat to the head of the frogs. This allows the eardrums of the sap to pick up sound from outside the ear and also record the vibrations of the lungs.
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Previous research has suggested that this lung-to-ear connection could increase a frog’s ability to identify a potential partner’s call by providing additional sound input, but that hypothesis was not unleashed when Lee, who conducted the research at the University. of Minnesota, and his colleagues proved it. Instead, they found something even more unusual when they aimed a laser vibrometer, which measures vibrations at a distance, against frogs bombarded with sound waves in the lab.
When the researchers performed a series of sounds to the women, something strange happened between 1,400 and 2,200 Hz. Within that range, the swollen lungs of the frogs resonated with additional vibrations and the movement of the eardrum calmed down, averaging between four and six decibels.
“That’s a difference a frog would notice,” Lee says. Somehow, additional vibrations from the lungs cancel out sounds of the same frequency in the eardrum, reducing sensitivity in this range.
This drop in sensitivity falls right between the two most prominent frequencies of a male green toad’s cry, suggesting that swollen lungs do not affect a female’s ability to hear her own species. But diving coincides with the dominant frequency of five species commonly found in the same ponds, such as bull frogs and barking frogs. The accuracy of the silent lungs of these sounds in the eardrum is not yet clear, but the net effect is a significant reduction in ambient noise that allows females to focus on important calls.
“I was almost overwhelmed by this work,” says Mike Ryan, an evolutionary biologist at the University of Texas at Austin. "It shows that the function of the eardrum is not static, but can be dynamically modified by the lungs in a way that reduces sensitivity to frequencies that are not 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 on the part of the brain. “This lung trick really cleans the sounds” before they reach the brain, Ryan says. "We don't think the lungs play a role in hearing, but the way this works is very, very cool."