American green treefrogs are able to pinpoint a mating call from a potential mate by inflating their lungs to reduce their eardrums’ sensitivity to other noise — including muffling other species’ mating calls — according to new research from the University of Minnesota and St. Olaf College.
Published in the journal Current Biology, researchers used precise laser measurements of the frog’s eardrums in response to sound to investigate the function of a lung-to-ear sound transmission pathway unique to amphibians. With their lungs in a naturally inflated state, the frogs take part in what’s called spectral contrast enhancement. This allows female frogs to limit their eardrums’ response to noise so they can better focus on the frequency range of the males of their species.
“Essentially, what female frogs are doing is putting on noise-canceling headphones,” said senior author Mark Bee, a professor at the University of Minnesota’s College of Biological Sciences. “Their lungs appear to dampen vibrations of the eardrum to help reduce picking up the mating calls of other frog species that take place at other frequencies.”
To discover this, researchers used data from the North American Amphibian Monitoring Program, which was a long-term, citizen science project that monitored amphibian populations across the eastern and central United States. Researchers chose to examine data from the American green treefrog, which has a high rate of co-calls with other frog species. Overall, about 42 different frog species co-call with the green treefrog, meaning they take part in a mixed chorus of mating calls. What the researchers’ analysis found was that when the lungs were inflated, it had no impact on how the green treefrog’s eardrum responded to its own species’ calls. However, inflated lungs reduced the sensitivity of the eardrum to sound frequencies commonly used by other species in multi-species breeding choruses.
Additionally, with the creation of a model of a green treefrog’s inner ear, researchers examined how the lungs' impact on eardrums might affect the frog’s neural responses to mating calls from their own species. Their findings suggest that the lungs help them more effectively “tune in” to their own species’ call frequency, allowing them to reduce neural responses caused by the calls of other frog species.
“While we started this project to investigate the directional hearing in frogs, our findings led us down quite a different path,” said Norman Lee, an assistant professor of biology at St. Olaf College. “We ultimately uncovered a completely new role for the lungs in vertebrate hearing that is similar to the technology found in some hearing aids, cochlear implants, and noise-cancelling headphones.”
Future studies will examine how widespread the lungs’ noise cancelling effects are among other species. Funding for this study was provided in part by the U.S. National Science Foundation.