A concert of croaking male frogs is a fierce singing contest, and he who has the sexiest voice gets to mate the most. Now two ecologists working in Malaysia report on a tree frog that has taken this musical battle to a strategic extreme: It adjusts its pitch to match the resonance frequency of the hole it lives in, thus tipping the scales in its favor.
Björn Lardner of the Field Museum in Chicago and Maklarin bin Lakim of Sabah Parks, a state park agency in Kota Kinabalu, Malaysia, spent more than 100 nights in the Bornean rainforest, locating calling males of the tree hole frog (Metaphrynella sundana). The 2.5-centimeter-long animals live in natural, water-filled holes in tree trunks, from whence they squeak every 5 seconds or so, says Lardner, who used ladders and mountaineering gear to reach animals that had taken up positions in treetops. The high-pitched beeps attract females that mate with the male and then lay their eggs in his tree hole.
The researchers noticed that some males would vary the pitch at which they called by up to 300 Hertz, whereas other individuals maintained the same tone throughout their recital. To understand what was going on, Lardner and Maklarin installed a plastic pipe in the forest, filled it halfway with water, and added a male tree hole frog. As the frog started calling, they slowly drained the water from the pipe. At first the animal varied its tone erratically. But as soon as it hit on the pipe's resonance frequency, which amplified its call, it would maintain that tone and start calling faster and with longer beeps. As the water drained and the resonance frequency of the "organ pipe" dropped, the frog lowered its pitch, closely tracking the ideal pitch until it lost it. Then, it resumed calling erratically again, the researchers report in the 5 December issue of Nature.
Lardner speculates that even though such fast, sustained calling exhausts the frog, it might pay off by bringing in more mates. Bill Etges of the University of Arkansas, Fayetteville, a specialist on male calling in insects and vertebrates, says the new study is "another example of how natural selection can fine-tune the acoustic attractiveness of frogs." But he cautions that the work should be followed up by observations on how many eggs the males in resonating holes actually fertilize.