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Planet Earth

Feathers That Sing: What A Little Sexual Selection Can Do

The LoomBy Carl ZimmerNovember 11, 2009 8:51 PM


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If you ever find yourself in the forests of Ecuador, you may have the good fortune of spotting a club-winged manakin. The closest the rest of us will probably ever get will be to watch this video. But don't just watch it. Listen. If you said to yourself, "Hold on, is that bird singing with its wings?" the answer is yes. As I wrote in this 2005 article in the New York Times, ornithologists have long known that a few species of manakins can make sounds with their wings. The sounds are produced by the males, as part of their courtship displays. Some make firecracker pops, and some make whooshing sounds. Darwin pointed to the sounds of manakin wings as evidence of just how much sexual selection could transform male animals as females were attracted to some mates over others. But no one had any idea how manakins could make noises with feathers until Kimberly Bostwick of Cornell and her colleagues tackled the question. Bostwick took a high-speed camera into the jungle to film club-winged manakins. It turns out the birds flap their wings 100 times a second, far faster than typical birds. Later, she closely examined museum specimens. Club-winged manakins have one peculiar wing feather with a stiff, curved tip, right next to one with a series of ridges. Bostwick and her colleagues proposed that curved tips raked across the ridges on the neighboring feather like a spoon pulled across a washboard, producing the bird's 1500-cycle-per-second sound. Biologists are quite familiar with this way of making sound--but in crickets and other insects. Typically, they draw their legs across ridges on their exoskeleton, making their bodies resonate in a process called stridulation. Bostwick and her colleagues were proposing, for the first time, that a vertebrate could stridulate, too. Since Bostwick published her first paper on the birds, she's continued to study them to test her hypothesis. In a paper just published in the Proceedings of the Royal Society B, she and her colleagues report a new experiment in which they looked at the physics of the manakin feathers. They clamped the feathers in a device known as a vibration mini-shaker, and then--well, as you can guess--shook them. The scientists bounded lasers off the feathers to track their wiggles as the mini-shaker vibrated faster and faster. They used the device first to measure the special spoon-and-washboard pair of feathers. Then they measured how other feathers responded, and then, finally, they studied a set of ordinary and spoon-and-washboard feathers joined together on a ligament. The scientists found that the spoon-and-washboard feathers resonated at about 1500 cycles, just as Bostwick had predicted back in 2005. The unmodified feathers on other parts of the wing, however, showed no such response when the scientists shook them one by one. But when they shook the spoon-and-washboard feathers together with seven neighboring wing feathers, the entire set resonated strongly at 1500 cycles. As you can see in this video, the club-winged manakin moves its entire wings upward before flapping. Bostwick proposes that the spoon-and-washboard feathers create a 1500-cycle sound, which the entire wing amplifies. This special kind of stridulation is not totally unique--the Australian whistling moth whistles by clicking castanet-like organs together, causing its entire wings to resonate. What is unique, however, is the evolution of feathers into such a sophisticated sound system.

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