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The behavior probably isn’t conscious—just something frog lungs are naturally able to do, via some unusual vibratory shenanigans. But biologically speaking, “it seems incredibly smart,” says Amritha Mallikarjun, a cognitive scientist at the University of Pennsylvania’s school of veterinary medicine, who wasn’t involved in the study. “They’re taking sounds that aren’t interesting, and trying to reduce them.”
Humans, to be clear, cannot do this, at least not with our lungs. The tubes that connect our ears to our upper throats are closed off most of the time, so most of the sound waves that reach our brains come in through the holes on either side of our heads. Almost all frog ears, however, are permanently linked to the rest of the body, and thus privy to vibrations from the mouth, the lungs—even the opposite ear. (This interconnectedness means that, unlike humans, frogs don't experience a pressure differential between the outside and the inside of their heads; their ears probably don't pop when they travel on planes.)
Scientists have been studying the open floor plan of frog ears since at least the late 1980s, when they discovered that the lungs were likely sending vibrations up to the animals’ heads. (A smear of Vaseline on the frogs’ torsos, others confirmed, could dampen the effect.) But the purpose of this bizarre connection eluded researchers for decades.
Jakob Christensen-Dalsgaard, a biologist at the University of Southern Denmark and an author of the new study, spent much of his career convinced that lung vibrations served as a sort of rudimentary GPS, helping frogs determine the direction that sounds were coming from and thus pinpointing potential mates in space. But it turns out that they’re filtering for quality, not location: When filled with air, the organs enhance the frog’s ability to tune in to only certain sound frequencies and cast others aside.
To tease these possibilities apart, the researchers put female American green tree frogs in an acoustic chamber and played the animals a mélange of sounds from different spots in the room. They then used a specialized laser to measure how much their eardrums vibrated in response.
For the experiments to work, Lee, of St. Olaf College, had to become a pro at deflating and reinflating frog lungs. Squeezing the air out, he told me, is pretty simple—a matter of gently squishing the skin around their lungs. (Frogs don’t have ribs.) Bringing the animals back up to size is trickier. For that, Lee inserts a segment of plastic tubing into the frog’s mouth and blows into the other end, transferring a teeny puff of air from his human lungs into the frog’s much smaller ones.
In the team’s experiments, puffed frogs seemed no better equipped than squished frogs to map out the sounds engulfing them. Inflation status also had little effect on the females’ ability to hear males of their own species (which, for the record, sound a bit like a goose honking in falsetto). But aerated frogs seemed worse at detecting noises in a very specific frequency range—one that happens to overlap with the calls made by several other amorous amphibians.