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Wooing Mates with Acoustic Tricks

The male tree-hole frog is a one-inch-long fellow that lives in the rainforests of the southeast Asian island of Borneo. He's not much on brainpower, but he's smart enough to do something no researcher has ever seen before, according to a new study in the journal Nature.

As NPR's Christopher Joyce reports, sex is behind it all. For this particular frog, reproducing means finding a fist-sized tree cavity with a small pool of water inside it. Then, to attract a wandering female, he sends forth a throaty little chirp.

The Bornean tree-hole frog's peep hardly makes him Don Juan on the trail of seduction. But it caught the ear of University of Lund biologist Bjorn Lardner during his long nights wandering the trails of the Borneo rainforest. He climbed up a tree to look into a tree hole, and an idea took shape.

"If you think of an organ pipe, if you imagine a frog sitting inside the pipe calling, if the frog calls with the same pitch as the organ pipe has, the resonance effect will cause the outgoing sound to be stronger," says Lardner, who conducted the research while at Chicago's Field Museum.

Every tree hole -- like every organ pipe -- has a particular volume and shape. And for that shape, there is small set of sound frequencies that exactly fit that shape, called its "resonance frequency." Hitting that frequency just right gives maximum volume.

The tree-hole frog peeps, then listens, then peeps again at a different pitch. He keeps changing his peep until he hits exactly the right pitch. The sound energy builds on itself, and then voila, the frog is transformed into Luciano Pavarotti.

To test this "self-tuning" ability, Lardner placed a frog in a plastic pipe with some water. The frog peeped. Lardner changed the water level and thus the volume of air in the tube. And each time he changed the water level, the frog changed the pitch of its call to match the tube's new resonant frequency.

When a frog hits the right pitch, Lardner says, he goes all out with longer and more frequent calls. And if he's lucky, there will soon be a knock at the door.

"The female sits at the edge of the hole peering down on the male singing. If she decides to mate, she will go down into that tree hole, he will grab her and they will lay their eggs there."

Lardner says the tree-hole frog is the only animal known to change its call on the spot to get the maximum volume boost out of its surroundings.

Cornell University behavioral ecologist Jack Bradbury says other animals also exploit the laws of acoustics quite ingeniously.

The mole cricket digs a burrow shaped almost like the horn on a bicycle, with a bulb on the bottom, a narrow neck and flaring horn. The design of the burrow boosts the volume of the cricket's call, says Bradbury. And the cricket will keep remodeling the burrow until it gets maximum volume.

Maximum volume, whether it's crickets or frogs or whales, leads to maximum sex appeal.

"Trying to attract mates is such a critical part for a species' survival," says Bradbury, "so if there's a way in which an animal can improve its range to attract more females, they are going to figure it out."

So long, of course, as they have a few million years to work at it.

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Christopher Joyce is a correspondent on the science desk at NPR. His stories can be heard on all of NPR's news programs, including NPR's Morning Edition, All Things Considered, and Weekend Edition.