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A pair of gray treefrogs in amplexus. Studies show that female frogs are able to pick out mates from a chorus at a great distance away.
To listen to a recording of a frog chorus or a Gray Treefrog call, go to the Imprints Web site and scroll to the bottom of the page.
Can frogs teach us how to build a better hearing aid?
Can frogs teach us how to build a better hearing aid?
By Kate Tyler
July 11, 2006
As the last traces of sun disappear on a balmy spring evening, Mark Bee heads out for a bit of Minnesota nightlife. Wearing hip waders and armed with a digital tape recorder, the behavioral biologist joins several student assistants knee deep in a pond in Carver Park Reserve, near Chanhassen. It's one of the area's best places to catch the chirping, croaking, trilling sounds of Minnesota's "frog chorus," which swells to deafening proportions during mating season (mid-April to early July).
A music enthusiast who gigged his way through college playing the electric bass and six-string guitar, Bee is not here just to groove on the melodies of hundreds of amphibians. He's conducting research for an intriguing project that may eventually help scientists develop a better hearing aid for human beings.
Bee is an assistant professor in the University's Department of Ecology, Evolution, and Behavior whose work focuses on the mechanisms and evolution of animal behavior, particularly acoustic communication. He's especially interested in the so-called "cocktail party effect"--the ability of many animals and most human beings to pick out a sound or voice from the acoustic clutter of many other voices and sounds. Anyone who saw the popular documentary March of the Penguins will recall the moving scenes of emperor penguins, returning from food runs to the distant sea, zeroing in on the calls of their mates and chicks even amid thousands of noisy--and identical--looking-penguins.
Bee's work on "acoustic signal recognition" during frog courtship aims to fill in details eagerly sought after by scientists working to improve human hearing aids and cochlear implant devices--"to learn the basic principles of how auditory and neural systems solve these problems," says Bee.
The frog chorus offers another great example of the cocktail party effect, says Bee. Over several decades, scientists have found that female frogs have an uncanny ability to identify the calls and exact locations of male frogs of their own species. Even with frogs of several species singing their hearts out in what Bee calls "a continuous din"--and sometimes against a backdrop of human-generated (anthropogenic) noise such as highway traffic--a female gray treefrog (Hyla chrysoscelis), for example, can find a mate amid the mayhem, ensuring that her eggs are fertilized. And, says Bee, some frogs can even learn to recognize particular individuals--Bob the bullfrog knows Harry the bullfrog's call, and responds aggressively if another bullfrog starts calling from Harry's territory.
What scientists don't yet understand are the precise mechanisms involved. Bee's work on "acoustic signal recognition" during frog courtship aims to fill in details eagerly sought after by scientists working to improve human hearing aids and cochlear implant devices--"to learn the basic principles of how auditory and neural systems solve these problems," says Bee.
Human beings with healthy auditory systems do just fine amid acoustic clutter--right down to picking out a friend's voice in a clamorous coffeehouse or pinpointing the sound of a neighbor's car coming down a busy street. But many elderly people and those with hearing aids are utterly lost trying to follow the swirl of conversation around a family dinner table.
"We need to know more about how to design auditory aids and implant devices that will excel at segregating voices from background noise," says Bee. "The frog chorus may have a lot to tell us about that."
Bee's visits to various area wetlands enable him not only to record the acoustic hubbub needed for close analysis of the frogs' "auditory scene," but also to gather frog pairs for study in the controlled environment of a University lab (after which they'll be returned safely to their ponds).
From pond to lab
On his nocturnal expeditions, Bee seeks out the frogs who've suddenly gone quiet. He'll generally find them tucked beneath floating leaves in amplexus, as scientists say--engaged in the mating embrace. Although he collects 15-20 pairs on a good night-usually right after a warm rain--it's tough to spot them, says Bee, whose sites include the Tamarack Nature Center in White Bear Lake and the Carlos Avery refuge near Forest Lake. The gray treefrog, one of 15 native Minnesota frog species, is a case in point. It measures only 21/2 inches long, and its gray-green sheen is nearly indistinguishable in a wetland.
"Usually we'll see just an eye or a nose sticking out of the water under some vegetation," says Bee, who uses his cupped hand to place the squirming pair, and a little pond water, into a ventilated plastic container. If the frogs mind the change of venue, they don't show it: they become unamplexed during the transfer, but most quickly resume mating. ("This time of year, they usually have one thing on their minds," smiles Bee).
About 2 a.m., Bee and his team of six undergraduate assistants bring his newly gathered frog pairs to the Animal Communication Lab on the St. Paul campus. Still in their ventilated containers, the frogs are refrigerated overnight to lower their body temperatures. "It temporarily prevents the females from laying eggs, so that they'll still be in mating mode for our experiments," says Bee, who is quick to note that the frogs--cold-blooded animals whose body temperatures reflect their environment--"are quite used to dealing with Minnesota cold snaps."
Like all good experimental researchers, Bee takes pains to control as many variables as possible. Brought out for a star turn in a specially constructed test arena, the female frogs first warm up in an incubator set to mimic their recent outdoor mating climate (11 C [52 F] for a spring night or 20 C [68 F] for a summer night). Then they're placed in a small release cage in the center of a circular sound chamber, two meters in diameter, with two-foot-high walls made of chicken wire and covered with black cloth.
Bee's sound-chamber experiments examine in minute detail how the female frog orients to the sound of a male frog's mating calls. The chamber is as dark as a starless night in a marsh, but audio speakers and an infrared camera allow Bee to monitor the arena both aurally and visually from an adjoining control room. Just before a pulley system lifts the lid of the release cage, Bee programs the speakers to play back a male frog call, a synthetically constructed version of the frogs' signature birdlike mating trills. (Only Pacific treefrogs--" Hollywood frogs," Bee notes wryly--actually say "ribbit.")
The pure digital signal allows Bee to manipulate such variables as amplitude (simulating sounds at various distances) and frequency (fine-tuning the call's "pitch"). Past studies have found female frogs able to pick out mates from a chorus at a great distance away. But do females orient to male frog calls faster at 20 meters than at 40? What if some calls are higher instead of lower, or sweep up slightly at the end? Researchers already have found that female frogs "find sexier than average those males whose calls are somewhat longer--we've dubbed them the 'machos,'" says Bee. From an evolutionary standpoint, he adds, the 'machos' turn out to be a good bet. "Their offspring are more robust--stronger, bigger, faster at getting out of a pond to evade predators."
Making the leap--from frog to human
Frog signals and mating calls are acoustically much simpler than human speech, but Bee intends his research to make the eventual leap from the frog pond to the human hearing research lab. Studying frog communication "could help us see something we might miss otherwise," says Bee.
"It's not that I'm going to publish a paper and a hearing aid company will read it and immediately design a better hearing device," says Bee, who eventually will study how frogs' brains make sense of sounds (his postdoctoral research was on the auditory neural physiology of European starlings). "But this work will help us better understand the basic principles involved."
The female frog in a chorus "has to solve the same basic problem as a human individual at a noisy party," Bee emphasizes. "Both have to sort through overlapping sounds, isolate peaks and valleys in different frequency ranges, and exploit different aspects of the acoustic environment to pick out relevant signals."
Bee's frog research will look squarely at the cocktail party effect--the phenomenon that most vexes many humans who wear hearing aids.
Bee finds that his test frogs invariably hop straight for any speaker broadcasting the digitally recorded call of a male of their species. But he's eager to study what happens when carefully manipulated background noise is added to the digital signals. Are there limits on frogs' abilities to pick out their own kind in a cacophonous mixed-species frog chorus? How might anthropogenic sounds such as that of traffic or overhead airplanes affect frogs' orientation toward mating calls?
From conversation to conservation
Beyond its implications for human hearing research, Bee's work promises to yield valuable insights for conservation biology. Although new frog species are still being discovered, the decline of frog populations worldwide has been well-documented.
"The No. 1 issue is habitat loss," Bee says, "mostly resulting from urbanization and sprawl."
Even if frogs survive the building of a highway next to a wetland, Bee says, "there could be a great impact just from the traffic noise alone. No one's looked at whether anthropogenic noise disrupts the frog's communication system, although we know that it has negative effects on other animals, such as songbirds and marine mammals." What if female frogs can no longer distinguish between 'machos' and 'wimps,' thus filling ponds with wimpier offspring? Would it make a difference if a highway were sited just a little farther away from a wetland? Are some noise frequencies worse than others in terms of masking frogs' acoustic signals--a passing truck worse than an overhead airplane?
"The more we know about the impact of anthropogenic noise," says Bee, "the better we'll be able to develop smart conservation strategies."
From Imprint, a publication of the Bell Museum of Natural History