Good vibrations: whether it's a croaking frog or a foot-stomping elephant, many animals use sound waves to keep in touch

Science World, April 26, 2004 by Britt Norlander

Noisy neighbors are keeping many residents of the Big island of Hawaii from getting a good night's sleep. And it's not a new rock band playing until the wee hours of the morning. Rather, it's a tiny frog--no bigger than a quarter--that's creating the racket.

Every night, male coquis searching for a mate make shrill "ko-KEE!" screeches loud enough to travel through the closed windows of homes and hotel rooms all over the island.

Their pint-size bodies are adapted to "speak up" so a potential mate can hear them. Like most species throughout the animal kingdom, coqui frogs communicate with one another using sound waves (vibrating energy waves that travel through a medium, such as a solid, liquid, or gas). From lizards to elephants, animals are specially suited to send and receive these vibrations so they can defend their turf and keep in touch with relatives.

LOUD MOUTHS

Populations of coqui frogs, which have no natural predators in Hawaii, have skyrocketed since the amphibian made its first appearance on the islands in the early 1990s. Scientists think the flogs, native to Puerto Rico, may have hitchhiked to Hawaii inside potted plants brought over from the Caribbean. "Today, there can be as many as 10,000 to 15,000 [coqui] flogs per acre in high density areas," says Bill Mautz, a biologist at the University of Hawaii. With so many coquis seeking mates, the noise level on the island goes sky high. "An individual call is just a two-note whistle," Mautz says. "But when high numbers of frogs call together, they create continuous noise." The noise levels can reach 70 to 75 decibels (measure of loudness), about the same as the roar of a garbage disposal. "It's like being in the middle of a noisy party," he says. "You can still heat', but you have to raise your voice."

PUMP UP THE VOLUME

Why the big communication commotion? Female coquis are attracted to the male that makes the loudest call. Just like humans and other animals, a coqui creates sound by blowing air out of its lungs. This thrust of air moves the frog's vocal cords (two pairs of folded tissue in the throat), causing air molecules to vibrate back and forth. Result? A sound wave.

To beat their competitors in the fight for females, these tiny frogs have a built-in volume control: an inflatable vocal sac (balloonlike pouch) beneath the mouth. To make a louder croak, many frogs inflate their vocal sacs while calling. They contract their bodies and squeeze air past their vocal cords into the sac. The air vibrates the vocal sac and produces a sound wave that has a larger amplitude (height) than a wave coming straight out of its mouth. The sound wave with file highest amplitude makes the loudest "ko-KEE."

CROAK TRANSLATOR

Coquis are good at belting it out. But still, most frogs are stuck singing a one-note tune. That's because compared with many animals, they have relatively primitive vocal cords. They can't change the sound wave's frequency (number of vibrations per second). Since the pitch (how high or low a note sounds) of a noise depends on the frequency of the sound wave, a frog's love time lacks variety. "They can't formulate songs like birds," explains Simeon McMillan, a 17-year-old high school student who studied frog calls for a research project that earned him one of 40 finalist spots in the national Intel Science Talent Search competition.

So how do other coquis know whether their neighbor is serenading them or telling them to buzz off? The meaning of each croak is hidden in its rhythm (pattern of beats). "The only difference between a distress call and a mating call is the rhythm pattern, or which beats are accented as they call," McMillan explains.

FEEL THE VIBE

Good communication goes beyond simply calling out to a neighbor. After an animal announces its presence, someone has to be on the receiving end of the call. How does an animal pick up a message? That depends: Some animals hear in surprising ways. When a sound wave hits a green anole lizard, for instance, the energy causes the lizard's chest wall to vibrate in tune with the wave.

Scientists recently discovered that this vibration travels through the animal's body and reaches sensory cells in the so-called inner ear (region hi the head that senses vibrations). Thomas Hetherington, a biologist from Ohio State University, explains that the reptiles don't have large chest muscles. Instead, their lungs are close to the body wall. "The sound energy probably flows along the respiratory pathways up to the head and then hits the inner ear," he says.

BOOMING BASS

For large creatures, it takes a powerful sound wave--with a high amplitude and low frequency--to create strong vibrations in the body and lungs. One big boomer: an elephant, which often rumbles with frequencies below 20 hertz (vibrations per second). These infrasound waves are below the hearing range of humans (see diagram, p. 16). While studying elephant calls m Africa, Caitlin O'Connell-Rodwell, a biologist at Stanford University, learned that even if she couldn't hear the animal, she could feel the vibration. "An elephant's rumble is so powerful that you feel it tilt you in your heart and lungs," she says.