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Making sense of animal sounds; scientists studying animal sounds are finding common threads to the barks, growls and whines of different species.

Tales of talking animals abound throughout history and literature. From the serpent in the Garden of Eden to the characters in Aesop's fables, people have been fascinated with animal talk. But can anyone imagine a bird that barks or a lizard that growls?

Ornithologist Eugene Morton can. In 1977, Morton came up with a theory of animal language that says, in effect, that almost all animals bark and growl. Although Morton has studied mostly birds, other researchers have applied his findings to mammals. Recent research on animal communication has both corroborated Morton's theory and uncovered exceptions to it.

Morton, who is with the Smithsonian Institution's National Zoological Park in Washington, D.C., calls his theory of animal communication the motivation-structural (MS) rules model. He talked about the model and other research in animal communication at a recent Smithsonian seminar on the subject.

The model holds that differences in the physical structure of animal sounds reflect differences in animals' motivational states. A frightened animal, for instance, makes a noise that not only sounds but also "looks" different from the sound made by an animal ready to attack.

Morton uses a sonagraph to study how sounds look. This device, about the size of a sewing machine, is composed of filters tuned to certain frequencies and piled on top of each other. The energy of a sound at a particular frequency creates a charge, and a stylus records a black mark indicating that frequency on a sheet of special paper.

The resulting sonagrams provide twodimensional pictures of individual sounds. That's where a bird's bark and a lizard's growl come in. In certain motivational states, a bird makes a sound that looks like a bark on a sonagram. It even sounds like a bark when slowed down and played back on a tape recorder. Likewise, a lizard's low, rumbling noise before an attack looks like a growl on a sonagram.

Morton discovered two extremes of vocalization in his studies of animal sounds. He developed hypothetical sonagrams to describe these and other sounds. An aggressive animal makes a low, harsh sound (the "growl") that looks like a low-frequency, broad band on a hypothetical sonagram (see diagram). A friendly or fearful animal makes a high, tonal sound that looks like a thin line.

The bark is between the two extremes (center block in diagram). According to Morton, the bark's continuous rise and fall in frequency indicates that the animal is neither aggressive nor fearful or appeasing, but is "interested" in what is going on. A dog's bark when it hears a person approaching is perhaps the best example. The bark indicates that the dog is not yet committed to a motivational state. If the approaching person turns out to be the mailman, the bark quickly changes to a growl; if the person is the dog's owner, it becomes a whine of acceptance.

Human study of animal sounds is difficult, Morton says, because humans rely on assumptions about their own language that may not be true for animals. When two people converse, the speaker informs the listener. Known as the information theory, this basic scheme, speaker [right arrow] message [right arrow] listener, has dominated communication studies for 25 years. Animals, however, can communicate even when no receiver is there, Morton says.

For instance, when a bird is caught by a hawk, it gives a distress call, not because it is trying to attract a potential savior (although it may do just that) but because after many similar episodes among the bird's ancestors, natural selection determined that the call is the best behavior under the circumstances. Although the bird is unlikely to escape, a small percentage of birds who give distress calls do escape. The hawk may be startled and let the bird escape; a larger hawk, attracted by the sound, might come along, and the bird might escape in the competition over the prey; or members of the bird's species may try to mob the predator, letting the bird escape.

Information theory does not allow for such an evolutionary perspective of animal communication, Morton says, "Information theory is a little too far advanced, too far from evolutionary-based communication," he says. "It doesn't deal with structure and function." Yet the tenet that structure can predict function is a fundamental concept of biology. Sonagrams of animal sounds, he says, show that the concept holds for animal communication as well.

Human speech relies on arbitrary words to convey meaning, Morton says, leading us to assume that animal sounds are arbitrary as well. But the physical structures of animals sounds are not arbitrary. In fact, sonagraphic study of human speech reveals that it has a similar nonabitrary component: the pitch of a human voice. When a person is expressing a demand or aggression, pitch tends to decrease, but when a person is speaking to a baby, it increases. So when words are disregarded, human speech patterns follow the rules of animal communication, Morton says.

Biologist Peter August, who has also done research on the motivation-structural rules, agrees. "When you think of the MS rules in human terms, you find several fundamental noises they make that seem to jibe with them," he says. "When you open a cabinet and see a spider, you shriek with fear, a high-frequency, very tonal sound. The aggressive noises of football players on a football field are very different."

August and graduate student John Anderson, both of the University of Rhoda Island in Kingston, tested Morton's theory in 1984. From the theory they predicted that aggressive sounds should have a statishtically significant lower frequency and broader bandwidth than fear or appeasement sounds. They examined the literature on animal communication in 50 kinds of animals, from shrews to elephants, and found this to be true in most cases. "The motivation-structural rules," August says, "are one robust theory that seems to explain the vocalizations of many different kinds of animals."

But the literature review uncovered one important variation on the motivation-structural rules theory. The researchers found that mammals' fear and appeasement sounds vary more in band-width and frequency than predicted by Morton's rules. August says the discrepancy may be because fear and appeasement are two different motivations in mammals, whereas they are fused into one motivational state in the motivation-structural rules.

Edwin Gould, curator of the department of mammalogy at the National Zoo, has discovered another exception to the motivation-structural rules. In his studies of nocturnal animals and other animals that do not rely on vision as a primary means of communication, Gould found that repetition rate is more important than frequency range in expressing motivational state.

Many animals, including baby mice, bats, porpoises, shrews and the shrewlike tenrecs of Madagascar, are almost constantly producing sounds. The sounds provide a continuous set of signals for listening animals, who can gauge the animal's excitation indicator, as Gould calls it, by changes in the repetition rate.

A baby mouse, for instance, makes pulsing noises. If touches in an experimental setting, it pulses faster, showing that it has become more excited, Gould says. A baby bat lost from its nest increases the rate of repetition of its screeching calls. When the mother returns it to the nest, the bat decreases its rate of repetition.

Gould's excitation indication is similar to Morton's motivational state. Take the barking dog example. The dog increases its rate of barking when someone is approaching, showing that it is more excited. "The dog is fearful and aggressive at the same time," Gould says. "This fits in quite well with Morton's rules."

Just as Morton's motivation-structural rules have human applications, so do Gould's repetition rules. Humans have pauses during their speech, and animals--even ones that generally produce sounds continuously -- have pauses in their communication. Psychologists who study speech patterns often look at the pause patterns of human speeck. They have shown that the pattern and timing of pauses often reflect an individual's emotional state. Gould believes pause patterns in the repetition of animal sounds likewise reflect animals' excitation states.

But certain animal sounds have no equivalent in human communication. Among these are nonvocal sounds like the hiss, which are produced without using the vocal cords. The vocal cords are coupled to the brain, Morton explains, and thus vocal sounds reflect an animal's motivational state. A nonvocal hiss, however, is made by an animal that is frightened but wishes to stay where it is until more information about the situation is available. "The hiss provides a way to mask motivation but still look potentially threatening," Morton says. "If vocalized, the hiss would look like a fear sound on a sonagram."

Is a hissing cat playing a game of deception? Not at all, Morton says. "Natural selection has selected for the behavior," he says, "because on the average, it has beneficial consequences." That is, sometimes the cat scares the potential attacker away.

A cat's arched back and raised fur add to the effect of its hiss, as any cat owner knows. These behaviors make the cat look bigger and more threatening. They constitute a general evolutionary trend, Morton says: In the face of incipient attack, the bigger you appear, the better off you are.

The trend is apparent not only in visual displays, such as a cat's arched back or the frilled neck collar of a lizard, but also in animal sounds. The larger an animal is in comparison with others of its species, the lower, harsher and more threatening it sounds.

Amphibians like frogs and toads have it made: They grow all their lives, so the sounds they make constantly get lower and harsher, providing a true reflection of their size. A mating male frog, for example, can tell the size of an encroacher by comparing the encroacher's croak with its own. If the encroacher's pitch is higher, the mating frog is safe from attack because the other frog is likely smaller. If it is lower, the mating frog gets its chance to escape from a frog that is probably larger. Either way, the big frog wins.

Mammals and birds, which evolved later than amphibians, do not grow all their lives, so they are about the same size as others of their species when competing for mates. They evolved voice variability to compensate, Morton says. An animal that makes a low, harsh sound can ward off a potential intruder because the low voice makes it seem bigger. On the other hand, an animal that makes high, tonal sounds can express friendliness or fear.

Mammals and birds, then, appear to be quite clever in their uses of acoustic communication. But Morton is quick to point out that it probably has nothing to do with conscious intent. "Natural selection can come up with clever things," he says, "It's not the animal who's trying to be clever."
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Author:Bennett, Dawn D.
Publication:Science News
Date:May 18, 1985
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