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Science World

Good Vibrations: Whether It's A Croaking Frog Or A Foot-Stomping Elephant, Many Animals Use Sound Waves To Keep In Touch

By Britt Norlander

April 26, 2004
Copyright © 2004 Gale Group Inc. All rights reserved.
Copyright © 2004 Scholastic, Inc. All rights reserved.

ISSN: 1041-1410; Volume 60; Issue 13

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.


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."


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."


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.


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.


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.

This forceful sound wave helps the animal communicate with elephants roaming far away. Higher frequency sound--like a bird's chirp--scatters and dies out when the waves hit objects like trees. An elephant's low-frequency rumble, which can have a very long wavelength (distance between a wave's peaks), doesn't break down as easily.

How far an elephant's rumble carries varies according to the weather. Wind and heat--conditions when air molecules are farther apart--cause sound waves to scatter. "Under ideal conditions, when there is cooler air near Earth's surface, an elephant's rumble can travel about 10 km [6 mi]," says O'Connell-Rodwell. That's compared with 4 km (25 mi) under hot or windy conditions.


O'Connell-Rodwell believes that in bad weather, elephants send and receive vibrations through the ground as well. Particles in solid ground easily bounce back and forth as sound waves pass through. Air molecules, on the other hand, can get spread out by a vibrating wave, causing the sound to break down. Seismic waves (vibrating energy waves that travel through ground), created when an elephant stomps its feet or bellows loudly, cam travel 32 km (20 mi).

O'Connell-Rodwell noticed that to pick up the call, the animals used their legs: "They freeze, lean forward, and press their feet into the ground." She suspects that other large animals may also use seismic waves to communicate over long distances. When an animal can't see its nearest neighbor, vibrations are a great way to stay in touch.

It's Your Choice

1 The pitch of a noise depends on the

A. sound wave's amplitude.

B. frequency of the sound wave.

C. sound wave's intensity.

D. rhythm pattern.

2 In which of the following conditions does an elephant's call travel farther?

A. When air molecules are farther apart

B. Hot weather

C. Windy and stormy weather

D. When there is a layer of cooler air near Earth's surface

3 Which of the following is NOT true about a coqui frog's call?

A. Male frogs use it to attract a mate.

S. It can reach noise levels of 70-75 decibels.

C. Female frogs are attracted to quieter calls.

D. It is a nuisance to many Hawaiian residents because of the large numbers of coqui frogs.

4 According to the article, sound vibrations can reach the inner ear of a green anole lizard

A. through the ground.

B. by traveling through its respiratory system.

C. only if the lizard's body is under water.

D. through its vocal sac.

Lesson Plans


Animal Talk by Tim Friend, Free Press, 2004.

To learn more about Caitlin O'Connell-Rodwell's elephant research, read "Four Ears to the Ground," Natural History, April 2002.

Want to hear what some animals sound like? Tune in to their calls here:

Here's a great Web site about how sound waves act underwater. It includes reformation about how marine animals make and use sound. You can also hear sound clips of humans and animals underwater:

Did You Know?

* Coqui frogs create more than just noise pollution in Hawaii. "They feed on anything that crawls in front of them and fits in their mouths," says University of Hawaii biologist Bill Mautz. With large numbers of frogs invading the Hawaiian rain forests, Mautz fears for the survival of native spider and insect populations.

* In his award-winning science project, Simeon McMillan studied the calls of more than 200 frog species. He discovered that the frequency of their calls is an inherited trait: Each species uses one preferred frequency when they call. And within each genus, he found that the frogs' calls fell within a shared frequency range. His research gives scientists a new noninvasive technique for telling species apart.

Cross-curricular Connection:

Health: A decibel is a unit that measures the intensity of sound. Have students research the decibel levels emitted by some everyday encounters. For example: television, stereo, traffic noise. Then use tiffs Web site to lead a discussion on developing healthy hearing habits:

Critical Thinking: Many Hawaiians are debating about whether or not to exterminate the coqui frog population on the islands. Research both sides and stage a debate on the issue.


Good Vibrations

Directions: Match the word(s) in the left column with the correct phrase in the right column.

1. decibels a. number of vibrations per second
2. vocal cords b. balloonlike, sound-amplifying pouch
3. vocal sac c. head region that senses vibrations
4. amplitude d. sound waves that are below humans' hearing range
5. frequency e. measure of loudness
6. rhythm f. wave height
7. inner ear g. pattern of beats
8. pitch h. two pairs of folded tissue in the throat
9. infrasound i. vibrating energy waves that travel through the ground
10. seismic waves j. how high or low a note sounds


It's Your Choice

1. b 2. d 3. c 4. b

Good Vibrations

1. e 2. h 3. b 4. f 5. a 6. g 7. c 8. j 9. d 10. i

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