If you live near a pond, no doubt you've sat outside on a warm night and listened to the frogs ree-deeping. To us, it's a relaxing sound. To female frogs, it's downright sexy. The louder the chirp, the more interesting the male. So what's a Romeo frog to do if his voice isn't quite up to volume?
If he's a Borneo tree-hole frog, he may use the entire tree he lives in as an amplifier. This is the surprising finding of a team of researchers led by Bjorn Lardner at Sweden's University of Lund. Lardner found that tree-hole frogs will sit inside a hollow tree that is partially filled with rainwater and try out different pitches until they strike one that resonates with the tree itself. This may seem confusing, but as Lardner points out, any time you sing in the shower you will run across this effect. Certain pitches are much louder than others; some even seem to make the whole shower vibrate. That's the resonance effect.
Why shake the tree you live in? Because it turns your hollow tree into an amplifier. If you sing at just the right pitch, the tree itself reinforces your ree-deep with its own vibration, and sends out a tree-deep louder than any of your competitors. Other species, such as crickets, have been found to dig burrows of the right shape to amplify their calls, but only this little tree-frog has been discovered searching for the right pitch to make a tree shake. Try it yourself next time you're in the shower. With a little practice, you can sing like the sexiest frog in Borneo.