St. Olaf professor publishes research on natural noise-cancelling abilities of frogs
Active noise-cancelling headphones are a high-tech favorite of many college students, but as it turns out, frogs have had the technology for millions of years. In a new paper featured in the journal Current Biology, St. Olaf College Assistant Professor of Biology Norman Lee describes new findings that frogs’ lungs act like noise-cancelling headphones to help frogs filter out certain sound frequencies that may be part of the calls of other frog species.
The work is attracting widespread attention, with features in The Atlantic, Smithsonian Magazine, Science News, and New Scientist. Lee also discussed his work with Canada’s premiere science radio program, Quirks and Quarks. It was even highlighted on The Late Show With Stephen Colbert (the mention begins at 5:39 in this clip).
For decades, scientists have known that the lungs in some species of frogs vibrate in response to sound, and that those vibrations can be transmitted to their eardrums.
“The precise function of this lung-to-ear sound transmission pathway has been a puzzle for several decades,” explains Lee. “Now we know that the lungs may function to cancel the eardrum’s response to noise produced by some other frog species in a noisy mixed-species breeding ‘chorus’. What the lungs are doing is called ‘spectral contrast enhancement’ because it makes the frequencies in calls from their own species stand out — compared to the noise at adjacent frequencies that are often part of other frog species’ calls.”
It may sound bizarre: frogs’ lungs enhance their hearing. Until recently, biologists actually hypothesized that a frogs’ lungs played a large role in helping them determine the direction of calls coming from frogs of the same species. However, Lee’s recent work along with collaborators Drs. Mark A. Bee (University of Minnesota) and Jakob Christensen-Dalsgaard (University of Southern Denmark) published and featured on the cover of the Journal of Experimental Biology dismantled this long-held view.
Building on this previous work, Lee and his collaborators now show that “when the frog’s lungs are under natural air inflation, they change the response of the frog’s eardrums to sound in a way that reduces their ability to hear some sources of noise.”
To reduce noise, “the lung’s resonance essentially records outside sounds in a specific frequency range, and then feeds this sound energy forward into the mouth cavity, Eustachian tubes, and middle ears,” Lee says. From here, the scientists hypothesize that the sound is fed back into the ear so that the frogs can cut through the noise and hear the specific frequencies that matter: mating calls.
Lee points out that this is something audio engineers deal with every day, especially when working with human speech recognition. But frogs figured it out first — probably more than 200 million years ago, soon after the first terrestrial vertebrates began living on land.
Our results stand as a reminder of the importance of basic research in integrative organismal biology, because evolution has a knack for hitting upon strategies for solving problems that human engineers discover only much later.Assistant Professor of Biology Norman Lee
Lee is excited to continue this line of research, and he hopes to figure out exactly how this process works and how wide-spread this ability is. This research comes out of the “Lee Lab” on the St. Olaf campus, where Lee recruits biology students to study the acoustic communication of insects and frogs. Last year, Lee collaborated with students in the lab to publish an article in the journal Frontiers in Ecology and Evolution on cricket song recognition by parasitic flies that hunt for crickets. When he’s not on campus, Lee may be found teaching “Island Biology in the Bahamas,” a popular Interim course at the Gerace Research Center on San Salvador Island.
“Our results stand as a reminder of the importance of basic research in integrative organismal biology, because evolution has a knack for hitting upon strategies for solving problems that human engineers discover only much later,” says Lee.