Frogs play tree: male tunes his call to specific tree hole.

Borneo's tree-hole frog may come as close to playing a musical instrument as any wild animal does, according to new tests.

Plenty of animals make structured sounds, but this inch-long rain forest frog adjusts its vocal performance to create a specific quality--resonance--from an object in its environment, says Bjorn Lardner of the University of Lund in Sweden.

That object is a tree with a cavity holding a puddle of water. Courting males of Metaphrynella sundana sit partly submerged in these puddles while chirping nighttime advertisements for females. When starting his call, the male raises and lowers the pitch until it hits the frequency that resonates in his particular cavity. Then he lengthens individual calls and shortens the time between them as he settles down for serious chirping, report Lardner and Maklatin bin Lakim of Sabah Parks Research and Education Division in Malaysia.

"If they gain a resonance effect, which is a bonus, they take the opportunity to invest even more energy in their calling efforts so as to become supersexy males," says Lardner

The researchers say in the Dec. 5 Nature that as far as they know, these frogs rank as the first examples of animals that test for resonance and alter their calls accordingly. "They do this actively--that's the interesting part," says Lardner.

Certain crickets and burrowing frogs build amplifiers, but they adapt their instrument instead of their performance for peak sound, Lardner notes. For example, mole crickets dig burrows of a size that resonates to their calls. Other crickets use cut leaves to amplify broadcasts.

To study the tree-hole frog, Lardner and bin Lakim recorded hundreds of calling sessions during Bornean-jungle nights. "We didn't realize what was going on at first," Lardner says. The crucial clue came near the end of their project when they put a male frog into a partially flooded artificial cavity made from a pipe. As the male started to chirp, the researchers gradually drained the water. When analyzing recordings, the researchers noticed a distinctive pattern of varying pitches produced until the frog hit the resonating frequency of its niche. As water dripped away, the frog matched the resulting changes in resonating frequency.

When Lardner and bin Lakim reviewed their earlier recordings, they detected similar patterns in frogs in natural settings, they report.

Lardner says that a person can imitate the courting frog by humming in the shower until the stall resonates.

Animal-acoustics researcher Michael Greenfield of the University of Kansas in Lawrence says that if the tree-hole frogs indeed use acoustic feedback to adjust their calls, they are unusual. People can easily listen to their attempts to sing a tune and then correct pitches, but "there's no evidence for that in insects, and I don't know of any in frogs," says Greenfield. "That's what's special here."

Greenfield cautions that more research will be necessary to figure out whether the frogs use solely acoustic feedback or get clues from other senses. Even if their feat is not exclusively acoustic, Greenfield says, "it's still neat."