In order to identify potential partners of their own species in the chorus of croaking frogs, American tree frogs have developed an extraordinary strategy: if they inflate their lungs, this reduces the sensitivity of their eardrum to croaking noises of other species. The calls of the potential mating partner can be heard more clearly. A similar principle is used in some hearing aids and cochlear implants, which are intended to enable their wearers to understand voices despite the noisy environment.
If a male frog is looking for a partner, he tries to lure her with loud croaking. But many different species croak in the choir of frogs. In order to be able to reproduce successfully, it is therefore important for the females to be able to distinguish males of their own species from those of other species on the basis of their calls. But how do they manage to hear and localize the right calls in the background noise of the frog concert?
Lungs as a silencer
Researchers led by Norman Lee from St. Olaf College in Minnesota have now discovered that the frogs’ lungs play a decisive role in this. With the help of a laser vibrometer that records vibrations without contact, Lee and colleagues measured the reactions of the eardrum of 25 American tree frogs, to which they played the croaks of various males. The females’ lungs were sometimes inflated, sometimes emptied.
The result: if the lungs were filled with air, the eardrum responded less to calls from alien males. The female frogs, on the other hand, always heard the croaking of males of their own species with the same loudness regardless of their lung status. “Essentially, the lungs cancel the eardrum’s response to noise, particularly some of the sounds that occur in a cacophonic chorus at reproductive time when males of different species croak at the same time,” explains Lee.
Parallels to modern hearing aid acoustics
In the art, this principle is known as spectral contrast enhancement. In the spectrum of different noises, certain frequencies are attenuated, others are emphasized. This method is also used for cochlear implants and some hearing aids, for example. This should make it easier for the wearer to be able to follow conversations even when there is a lot of background noise. “In hearing aids, these algorithms are usually designed to amplify the frequencies found in human speech or attenuate other frequencies, in some cases both,” explains co-author Mark Bee of the University of Minnesota-Twin Cities.
To better understand how the tree frogs achieve this effect, the researchers created a physiological model of the sound processing in the frogs’ inner ear. Accordingly, it is already built in such a way that it responds best to calls of its own kind. This coordination is refined by the lungs: the filled lungs resonate in response to sound and transmit the vibration to the inner ear. Such an acoustic connection is only found in amphibians. The vibration of the lungs creates a kind of counter-sound that suppresses frequencies that do not correspond to the calls of its own kind. “We believe the physical mechanism is basically similar to how noise-canceling headphones work,” says Bee.
Possibly common in other frog species as well
The researchers assume that not only American tree frogs use this tactic of selective hearing, but also many other frog species. But how does the lungs “know” which frequencies to filter out? According to the authors, both the quack frequency and the lung vibration of the frogs are related to their size and lung volume. Refined mechanisms of evolution have led to the lung vibration and croaking frequency of the respective species complementing each other optimally.
“The results show the amazing ability of evolution to use existing adaptations for new functions,” write the researchers. “This example also shows how nature takes precedence over human engineers when it comes to finding clever solutions to widespread problems.”
Source: Norman Lee (St. Olaf College, Northfield, Minnesota) et al., Current Biology, doi: 10.1016 / j.cub.2021.01.048