Gators go a-courtin’ with fancy physics

Subsonic mating bellows generate Faraday waves

Alligators flirt with physics. When males attract attention by quivering their spiky backs underwater, they create Faraday waves, researchers reported May 23 in Seattle at a meeting of the Acoustical Society of America. These sophisticated patterns are usually seen only in man-made devices.

“Faraday waves haven’t really been seen in nature before,” says John Allen, an acoustician at the University of Hawaii at Manoa who was not involved with the research.

When a male alligator craves company, he issues a sound from his lungs that is too low to be heard. This infrasound causes him to vibrate violently and whips the water on his back into a froth of waves and leaping fountains.

Acoustician R. Glynn Holt of Boston University saw a video of this “water dance” and was reminded of physicist Michael Faraday. In 1831 Faraday discovered that liquid above a vibrating object sometimes forms surface waves that move up and down at half the speed of the vibrations.

In search of Faraday waves, Boston University undergraduate Peter Moriarty traveled to Gatorama, a roadside attraction in Palmdale, Fla. He played alligator calls to a gator named Mr. Chicken and recorded the enthusiastic response, an 18- to 20-hertz rumble. Hidden in this sound was the telltale sign of a Faraday wave: another sound at half the vibrational frequency, 9 to 10 hertz.

Surface waves with this frequency should have a wavelength of about 3 centimeters — which agrees with measurements made from photos of amorous alligators. Three centimeters is also almost exactly one-third the distance between the rough protrusions, or scutes, on the creature’s back.

“We think that the shape of their scutes helps them create these waves,” said Moriarty. The shapes of some backs may make better waves for signaling mates, he speculates — possibly a kind of sexual selection.

How rough do alligators like it? Moriarty has built his own mock alligator back to find out.

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