Frogs have solved the cocktail party problem

When people attend large, noisy gatherings, hearing a conversation is often difficult. The so-called cocktail party problem also affects frogs in habitats teeming with the sounds of other animals and anthropogenic sources of noise. To find the signal from their mates in the noise, some frogs inflate their lungs to act as resonators that amplify certain characteristic frequencies. Others abandon acoustic communication altogether and resort to visual cues, such as waving their legs. (For more about acoustic biology, see the article by Megan McKenna, Physics Today, January 2020, page 28 .)

Some frogs, however, appear capable of solving the cocktail party problem with a noise-canceling acoustic technique. Norman Lee of St Olaf College in Minnesota, Mark Bee of the University of Minnesota, and their colleagues have now found that when frogs inflate their lungs, incoming acoustic signals from a biologically noisy frequency band are attenuated. The practice allows female tree frogs with inflated lungs to better hear the mating calls of their male counterparts.

For decades, biologists have known of the lung-to-ear sound-transmission pathway in frogs, so Lee and his colleagues used laser Doppler vibrometry to measure their ear vibrations with and without having their lungs inflated. The researchers initially expected the inflated frog lungs to affect the directional response of one eardrum. But to their surprise, they found that the lungs had no effect on the frogs’ directional hearing. Because frogs use the amplitude difference between eardrums to localize sound, the improvements in directionality at each eardrum canceled each other out.

Instead, the inflated lungs attenuated the sensitivity of their eardrums. The amplitude of the vibrations in the ears decreased an average of 4–6 dB when the lungs were inflated. The sensitivity loss was most pronounced in the frequency range between 1400 Hz and 2200 Hz. That band, the researchers found, is quite noisy: An analysis of a data set of multispecies choruses collected by the North American Amphibian Monitoring Program shows that many frogs use that frequency band for mating calls. Although the amplitude of tree-frog mating calls peak on either side of that frequency range, the noise can make finding a potential mate challenging.

That evolutionary pressure may have caused green tree frogs—and perhaps other frog species—to develop the noise-reducing adaptation for that frequency band. Lee and his colleagues aren’t entirely sure yet of the precise mechanism that causes the vibration attenuation, but they suspect it may be similar to the destructive interference used in noise-canceling headphones. (N. Lee et al., Curr. Biol., 2021, doi:10.1016/j.cub.2021.01.048 .)