How Female Frogs Tune Out Useless, Noisy Males

Before frat parties, there were frog ponds.

Literal breeding grounds for some of the world’s noisiest bachelors, these lusty pools are where amphibians gather to woo mates. And as any frog researcher will tell you, they’re “super, super, super loud,” says Valentina Caorsi, a bioacoustician at the University of Trento in Italy.

Some spots host hundreds of males from a dozen species, each belting out serenades that can register at more than 100 decibels apiece—close to what you’d hear at a rock concert or a rowdy nightclub. Sounds this intense can cause hearing loss; the National Institute for Occupational Safety and Health recommends limiting exposure to such cacophony to less than 15 minutes a day. When scientists visit these ponds, they often don earplugs. “It hurts our ears,” says Kim Hoke, a biologist at Colorado State University.

If frogs of the female persuasion can’t identify their own species’ calls in this terrifying soundscape, they may lose out on an opportunity to reproduce. Fortunately, evolution has come up with a clever trick to cut through the chaos: a pair of lungs that can help female frogs home in on the come-hither calls of potential mates, according to a study led by Norman Lee of St. Olaf College in Minnesota, published today in Current Biology. Remarkably, the lungs accomplish this not by amplifying the sounds made by males of the same species, but by muffling the ruckus of other species. Frog lungs, Lee’s team has shown, are basically noise-canceling headphones that also happen to oxygenate the blood.

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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.

Frog lungs, Lee and his colleagues discovered, vibrate within a range of about 1400 to 2200 hertz; the American green tree frog calls outside this range, while many of the toads and bullfrogs that share its habitat call within it. When these other species croon, female tree frogs’ lungs quiver, transmitting energy to the inner surface of the eardrums and instructing them to ignore similar sound waves they receive from the outside. It is, in a sense, an anti-eavesdropping device.

Lung-based interference can’t eliminate all the hubbub that females deal with in their environment. The maximum knockdown Lee’s team measured in the study was about 10 decibels. (For comparison, decent noise-canceling headphones can eliminate some 30 or 40 decibels of noise.) Still, “it’s an enormous bump down,” Mark Bee, a biologist at the University of Minnesota and an author on the study, told me. Frogs also might not want to completely deafen themselves to other sounds, which could alert them to the presence of predators or tasty prey.

The frogs’ ruse isn’t just impressive, experts told me. It’s also an elegant solution to a complex problem. In the world of acoustics, there are two ways to make a sound more audible. One is to boost the signal itself, in the hope that it will rise above the din. The other is to pare away the noise around it—a way of emphasizing the message without changing its contents.

Filtering out the nonsense of irrelevant males up front might also reduce the workload on frogs’ teeny brains, says Hoke of Colorado State University. When humans congregate at raucous parties or bars, they must devote a lot of neural resources to focusing on a single speaker at a time. What frogs have cooked up is a sexual sieve: “They don’t need to waste their resources processing it out,” she says.

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The study also highlights an oft-neglected part of courtship acoustics. For decades, scientists have meticulously documented the ways that male amphibians—ribbity Romeos that they are—change their behavior to make their voices stand out. Some croak, grunt, trill, and even squeak faster or with extra gusto; others will boot rival frogs out of their territory so they can hog the swampy stage for themselves, Bee told me. A few polite souls actually take turns to avoid getting drowned out.

But we don’t know nearly as much about what happens after signals are emitted, says Mariana Rodriguez Santiago, a neuroscientist at Colorado State University who wasn’t involved in the study. “The attention of academia has been very broadly focused on males and how they change what they’re doing,” she told me. Across the tree of life, females are often reduced to inert receptacles, or mere measures of males’ reproductive success.

That couldn’t be further from the truth, especially among tree frogs. Communication is a two-way street, and studies that focus on the female perspective—as this one did—are a powerful reminder that signal interpreters aren’t passive, Rodriguez Santiago said. Desperate messengers can scream and shout all they like, but none of it matters if no one hears them.