Female American green tree frogs use their inflated lungs to dampen the mating calls of other species so they can pick out the ones from males they may mate with.
Male frogs use mating calls, ranging from high-pitched cackles to deep croaks, to advertise themselves to nearby female frogs. But grabbing their attention means competing with the cacophony of calls from other frog species living in the same pond.
To find out how they navigate the noise, Norman Lee at St. Olaf College in Minnesota and his colleagues played a range of sound frequencies to 21 female Hyla cinerea frogs. They immobilised the frogs and then either inflated or deflated their lungs using a laser vibration sensor. A laser was beamed at a reflective bead placed on a frog’s eardrum. By measuring the laser that was reflected back, the team could estimate the amount of vibration at the eardrum’s surface that occurred in response to the sounds.
The frogs’ eardrums vibrated less when their lungs were inflated, but only for sounds within a specific frequency range. The background noise was filtered out when it fell between 830 and 2730 hertz, leaving audible those two frequencies – the same peaks found in the male H. cinereamating calls.
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“The call is a single-note call, it sounds like a cross between a dog barking and a duck quacking,” says Lee. When females hear that, it takes precedence over noise at the same frequency made by other species.
This would come in handy in the wild, where frogs and toads loudly call at the same time.
As this only occurs when the lungs are inflated, the team suspects that the lungs work in a similar way to noise-cancelling headphones, which use microphones to record the noise around you and then produce an exact opposite signal, known as the antiphase, to cancel it out.
“We think this is how it probably works in the lungs,” says team member Mark Bee at the University of Minnesota. The lungs seem to produce the antiphase signal to filter out other sounds in this frequency range, he says. The external sounds in this specific frequency range cause the lungs to resonate, producing a vibration that is exactly opposite and therefore cancels it out.
“This basic idea of cancelling out the noise between informative signal peaks is something that human engineers have been doing to improve hearing aids and cochlear implants for years,” says Bee. Next, they want to investigate if male frogs also have this ability and if any of the other 7200 frog species can too.
Journal reference: Current Biology, DOI: 10.1016/j.cub.2021.01.048