Scientists have discovered that by inflating their lungs, frogs can diminish their sensitivity to certain sound frequencies, making it easier to hear calls from their own species while tuning out those of others.
The study, published Thursday in Current Biology, uncovers an evolutionary solution for a classic struggle in sensory biology known as the “cocktail party” problem. Like trying to hear a single voice in a crowded cocktail party, female frogs have to parse out mating calls from males of their species within a cacophony of similar frog calls, some as loud as a lawnmower.
“Imagine a hundred different alarm clocks going off in your bedroom and having to pick out a specific one,” said first author Norman Lee, an assistant professor of biology at St. Olaf College in Minnesota. “This is what female frogs are up against.”
Studies from as early as the 1980s have reported that frogs’ lungs are intricately connected to their sense of hearing through a unique sound pathway between the lungs, middle ear, mouth cavity and Eustachian tubes, which run from throat to ear. But the nature of this connection has been a subject of debate among amphibian biologists.
A 1991 study led by Jakob Christensen-Dalsgaard, one of this study’s coauthors, suggested that the lungs play a role in directional hearing. However, Lee, Christensen-Dalsgaard and their colleagues have now determined that this is not the case.
“When you look at the response in just one eardrum, you do see a change in directionality in response to lung inflation, but in frequencies outside of the conspecific calls that the frogs need to localize,” Lee said, referring to calls made by the same species. “One thing that they really didn’t look at back then is that when frogs locate a sound, it involves both ears. When you start to look at binaural hearing, you find that the lungs really don’t improve directionality that much.”
Ruling out directionality, the researchers sought to uncover what purpose the lung-ear connection served in frogs, using the green tree frog as a model. To explore this question, the team used a method called laser vibrometery, using a laser beam to pinpoint how much the eardrums responded to sound vibrations at different levels of lung inflation.
They found that the inflated lungs resonated at a frequency that canceled out sound waves between the peak frequencies of the frogs’ mating calls. The effect, known as spectral contrast enhancement, is like that employed by noise-canceling headphones.
"In humans, these algorithms are designed to amplify or 'boost' the frequencies present in speech sounds, attenuate or 'filter out' frequencies present between those in speech sounds, or both,” said senior author Mark Bee, a professor of ecology, evolution and behavior at the University of Minnesota Twin Cities. “In frogs, the lungs appear to attenuate frequencies occurring between those present in male mating calls.”
Another part of the study examined what mating calls the frogs were competing against, to verify that this frequency-dampening effect would work on the other frogs in their environment. To do this, the researchers looked to citizen-science data from the North American Amphibian Monitoring Program, an initiative that trains people to distinguish and record calls from different amphibian species.
The team found that 42 species of frog live in the same area and co-call with green tree frogs, only 10 of which contributed to 80% of the observations of noise in the area. Of those 10 species, five had calls that fell within the frequency range dampened by the green tree frogs’ inflated lungs. The result is a massive reduction in noise for the frog when their lungs are inflated.
“We just thought, 'Wow that is so cool!'” Lee said. “The fact that the input of these lungs can reduce the noise of at least five of these other species was really exciting to us.”
The research is a shining example of a common theme in biology and engineering: Evolution often solves engineering problems before humans do, and nature is a potential goldmine of inspiration for engineers looking to solve the same problems.
“Spectral contrast enhancement is not a new concept to us. Engineers typically use it in sensory processing strategies in hearing aids and cochlear implants,” said Lee. “However, evolution has solved a lot of these problems hundreds of millions of years ago, while engineers have been working on them for just a couple of decades. There should be some focus on basic science, asking how evolution solved the problem, and maybe that can inform engineers in the work they do.”
The study, “Lung Mediated Auditory Contrast Enhancement Improves the Signal-to-Noise Ratio for Communication in Frogs,” published March 4 in Current Biology, was authored by Norman Lee, St. Olaf College; Jakob Christensen-Dalsgaard, University of Southern Denmark; and Lauren A. White, Katrina M. Schrode and Mark A. Bee, University of Minnesota Twin Cities.