When it comes to making sounds, size matters, at least to some bats. An oversized facial structure called a sella may help the Bourret’s horseshoe bat focus its sonar signals into a narrow beam, allowing the bat to sense faraway objects, researchers suggest in an upcoming Physical Review Letters. Understanding how shapes mold sound waves may lead to better acoustic devices, says study coauthor Rolf Müller of Virginia Tech’s Institute for Advanced Learning and Research in Danville.
In addition to preventing a Bourret’s horseshoe bat, Rhinolophus paradoxolophus, from winning beauty contests, convoluted face folding actually has an important job, Müller says. The bat’s extraordinarily long ear-shaped appendage, averaging 9.1 millimeters, protrudes from the front of a broader structure called a noseleaf, which sits above the bat’s nose. While some bats emit sonar signals from the mouth, Bourret’s bats emit sound from the nose, so the researchers thought that sellas may play a role in shaping the sound.
Müller, who conducted the study while at Shandong University in China, and his colleagues made computer models to see how sella size might affect sound emissions. “We could re-create what evolution has done in a computer and see what happens,” Müller says.
First, the team built 3-D models of three Bourret’s horseshoe bats’ heads. Using a new technique, the team then modeled the path sound waves would take as they leave each bat’s throat, travel through the nose, bounce off the edges of the sella and ultimately exit the body.
By varying sella size in the 3-D head models, the team found that short sellas widen the beam of certain frequencies of sound leaving the bat, whereas long sellas sharpen the beam. Müller likens the process to a circle of light from a flashlight. Twisting a ring on some flashlights can focus the beam from a wide, weak signal to a sharp, crisp dot. With a long sella, “the bat can do the same thing with sounds,” he says.
But longer sellas, the team found, can have diminishing returns. At a certain point, the lengthening didn’t provide as much sharpening bang for the bat’s buck. Coincidentally (or not), the natural length for all three specimens of bat fell right on this sweet spot. “The length where this change occurred is exactly the natural length,” Müller says.
Researchers don’t yet understand why Bourret’s horseshoe bats need a long, sharp sound beam instead of a wider, more diffuse beam. The longer beam may help the bat sense prey farther in the distance, Müller suggests. The tightly focused beam might also cut down on signals that bounce back from irrelevant objects in the bat’s southeastern Asian rainforest habitat.
Understanding how sound is molded by shapes may ultimately help researchers devise clever speakers and sensors, says bat acoustics researcher James Simmons of Brown University in Providence, R.I.
“This is very good physics,” Simmons says. “It just happens to be on animal parts.”
