Male frogs of many species rely on mating calls to attract mates. However, even after the female has fallen for the male with the sexiest ribbit, true love faces an obstacle: how to find the velvet-voiced charmer in amongst the noise. Some species have arrived at a truly remarkable solution – lungs that cancel out unwanted sounds so the amphibious Juliet can find her way to her froggy Romeo.
As anyone who has wandered past a suitable pond can testify, things can get pretty loud when the time is right for breeding. These sounds can come from many different species, creating an epic “cocktail party problem”. So how does a female tune out all the other noise and hop her way to her handsome hero?
Dr Norman Lee of St. Olaf College Minnesota found it's all in the lungs. When inflated, these help the females tune out background noise. “In essence, the lungs cancel the eardrum's response to noise, particularly some of the noise encountered in a cacophonous breeding 'chorus','” Lee said in a statement.
The lungs make the frequencies specific to the desired male's call stand out from background noise. "This is analogous to signal-processing algorithms for spectral contrast enhancement implemented in some hearing aids and cochlear implants," Professor Mark Bee of the University of Minnesota-Twin Cities said. “We believe the physical mechanism by which this occurs is similar in principle to how noise-canceling headphones work."
Human devices both amplify the frequencies used in human speech and filter out the rest. Frog lungs are only doing half the job, dimming frequencies between those used by the desired male, but that is still impressive enough.
The curious connection between frogs' lungs and middle-ears had been suspected to play a role in their mating processes, but herpetologists were not quite sure how. One theory was it helped with directional finding.
Bee and Lee tested this, and report in Current Biology that American green treefrogs are no better at assessing the direction from which a call comes when their lungs are inflated as when they are not. On the other hand, they found lung inflation affects the extent to which the frogs' eardrums vibrate in response to certain frequencies.
Specifically, male green treefrogs have calls that peak at 834 and 2,730 Hertz. Other species prefer different frequencies. By shining lasers on female green treefrogs' eardrums (a phrase we never expected to write), Bee and Lee confirmed the lungs reduce sensitivity to everything between 1,400 and 2,200 Hz. Other frogs that breed in the same locations use these in-between frequencies.
Of course, green treefrogs still have to cope with the sounds of species that go really low or very high, not to mention those quite close to that of their own males. Until someone invents frog Tinder, however, every bit helps.
Bee and Lee haven't extended their research to other species, but they note many frogs also have two, widely separated, peaks to their calls. They suspect other females are also tuning out an unwanted in-between range, the frequencies of which vary from frog to frog.