Frogs Use Hollow Trees as Megaphones

Sean Markey
National Geographic News
December 4, 2002
Chemistry may help keep a relationship together, but male tree-hole frogs found in the lowland rain forests of Borneo use physics to attract their mates.

Scientists studying the mating calls of Metaphrynella sundana have discovered that male frogs actively tune the pitch of their calls to resonate inside hollow tree cavities—their preferred mating habitat. Researchers likened the resonant effects to those of organ pipes. When successful, male frogs sound closer, louder, and presumably more attractive to prospective female mates.

Researchers also found that males will expend extra energy on their mating call—increasing the tempo and prolonging the duration of each call—to take advantage of the favorable acoustics.

"As far as we are aware, [this] is the first evidence that an animal can actively alter its behavior, its call pitch, to obtain…resonance…[to] produce this super-attractive call, which is quite complex," said Björn Lardner, one of two researchers to make the discovery.

Lardner, an animal ecologist at the University of Lund, Sweden, and his research partner, Maklarin bin Lakim, of the Sabah Parks Research and Education Division in Sabah, Malaysia, conducted six months of field research in Borneo's Kinabalu National Park. Lardner later analyzed audio field recordings and other data at the Field Museum of Natural History in Chicago, Illinois.

The discovery marks the first time an animal species has been shown to alter its calling behavior to exploit resonance effects. Some species of burrowing frogs and crickets are known to dig burrows or cut baffles in leaves to specific dimensions to create a resonant effect for their mating calls. But such adaptations involve the insect or animal changing their physical environment, not altering their individual calling behavior.

Since such behavior was not previously known to science, researchers have rarely looked for such traits in animals, Lardner suggested. The findings raise the possibility that similar overlooked animal behaviors await discovery, he said.

"The question that arises is, how common is this phenomenon?" said Lardner. "Is it that we haven't looked for it in animals? Could it be that animals are using [other] signals in more complex ways of exploiting the physics of the environment in which they live?"

An Unexpected Discovery

Borneo tree-hole frogs grow up to one inch (2.5 centimeters) in size and are endemic to the lowland rain forests of Borneo. They mate and breed in tree cavities formed by rotting broken limbs. Frequent rainfall can partially fill these cavities with water. Female tree-hole frogs lay their eggs in these miniature, sheltered standing ponds after mating.

To study the frog's mating-call strategies, researchers Lardner and bin Lakim recorded more than 300 mating call events by frogs in their natural habitat.

The pair also captured a male tree-hole frog and placed it in an opaque plastic cylinder partially filled with water. A drainage tube allowed the researchers to gradually lower the water level inside the cylinder to learn how the frog's mating call was affected by changing acoustic properties of the hole.

The researchers returned the following evening to record the mating calls of the male frog.

Analysis of those field recordings demonstrated that when male tree-hole frogs colonize a new hole, they can emit a series of calls to "sample" its acoustic properties. With each successive call, the frog can presumably adjust his call pitch to increase the volume it hears inside a tree hole. Male frogs eventually reach the resonant frequency of the tube if it lies within reach of their individual vocal range.

If resonance is gained, the male frog reaps a benefit. During their experiment, Lardner and bin Lakim found that the male frog increased the volume of its mating call by 10 to 15 decibels by leveraging the resonant frequency of the tube.

The physics of sound state that doubling the distance to the source of a sound decreases its recorded volume by six decibels. So the volume gained through resonance is significant, Lardner said.

"By exploiting the resonance effect, [male frogs] will appear to sit closer and/or they will sound more attractive to the female," said Lardner. "Not only will they be heard from a longer distance and attract females from a longer distance, but presumably—we can only guess—the female also appreciates a powerful, strong call as a sexy trait that indicates a powerful male." The researchers also found that when they drained water from the tube over a 28-minute period, the captive male frog changed the pitch of his mating call by up to 115 hertz to stay within the resonant frequency of the cylinder.

The vocal range of individual male tree-hole frogs vary, much like humans. Natural tree hole habitats also vary in shape, size, and resonant frequency. Luck therefore plays a role in whether or not individual male frogs can gain resonance in a given hole.

When male frogs do find a tree hole with resonant frequencies that lie within their vocal range, they pull out all the stops, Lardner said, increasing the calling rate and pulse duration of their mating calls.

"When they hear, 'Oh tonight I have a bonus effect because I am in a hole that is good, then I [will] really invest extra energy in these other call aspects," Lardner said.

An "Exciting Finding"

Ronald Heyer, a research zoologist with the division of reptiles and amphibians at the Smithsonian Institution's National Museum of Natural History in Washington, D.C., said that there have been a number of important discoveries involving frog calls in recent years.

Commenting on these latest findings on Borneo tree-hole frogs via e-mail, Heyer wrote, "This study adds yet another layer of understanding about communication in frogs, involving a completely unanticipated, unexpected, exciting finding."

"[It] seems to indicate that there is quite a bit of control over vocalization possible by individual frogs, which will make us have to again rethink what we understand about frog communication in general," Heyer wrote.

A summary of Lardner and bin Laksim's research appears in the December 5 issue of the science journal Nature.

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