Frogs Use Hollow Trees as Megaphones

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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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