Chimp "Dinner Conversation" Proof of Ape Speech?

Adrianne Appel
for National Geographic News
October 20, 2005
Scientists say they have discovered the first evidence that chimpanzees
speak to each other about objects in their environment.

Chimps at the Edinburgh Zoo in Scotland use a crude language of grunts to talk to each other about their food, say primate experts at the University of St. Andrews in Scotland.

The chimps utter high-pitched noises (hear the audio) or low-pitched grunts (hear the audio) to tell each other about the food they find in their pen, the researchers say.

The finding could lead to better understanding of the origins of human speech, the scientists say.

Previous studies have found that monkeys, as opposed to apes, communicate with each other through sound about events in their environment and that great apes can use hand signals. The new chimp finding, however, may be the first evidence of great apes using vocal communication.

Humankind's closest genetic relatives, the great apes include gorillas, chimpanzees, and orangutans.

Bread Good, Apples Bad

At the Edinburgh Zoo the chimps make high grunting noises when they find bread, a food they seem to like, and low grunting sounds when they find apples, which they apparently don't care for very much, according to the study, published last week in the journal Current Biology.

After noting the different types of grunts, the researchers set out to see if other chimps listening to the grunts interpreted them the way the researchers had ("bread" and "apple"). The researchers found that the listening chimp did seem to understand what the grunts mean.

The scientists recorded the grunts and played them to a chimp in the pen. When the chimp heard the "bread" grunt, the ape looked in the place in the pen where bread is usually found. When the "apple" call was played, the chimp searched appropriately for an apple.

"It shows that, by simply listening to each other's calls, chimpanzees can infer what kind of food the caller has found," said researcher Katie Slocombe, who worked with colleague Klaus Zuberbuhler on the project.

More Experiments Planned

"We don't know yet how specific these calls are—i.e., whether they specifically refer to bread or apples or whether they simply label highly preferred food [such as bread] and less preferred food types [such as apples]. We are planning further experiments to test these two possibilities," Slocombe said.

In research terms, the grunts are known as functionally referential signals: signals that animals give to each other in response to an outside event or object, such as an alarm call that warns of a predator.

"These 'rough grunt' calls are specifically produced when chimpanzees find food," Zuberbuhler said. "This study is special, because it provides the first evidence that listening chimpanzees are sensitive to this variation: They seem to understand that the calls refer to the food encountered by the caller," he added.

Debby Cox, the Jane Goodall Institute's executive director for Uganda, said the chimp grunting research is "just the beginning."

"It's a no-brainer that they're going to have high grunts for highly prized food. It's the same with people," Cox said from the institute's Arlington, Virginia, headquarters. "Chimps are very motivated by food. In any research we use food as a reward."

That's not to say the new findings aren't surprising. "If other chimps recognize the high grunts, that's something that hasn't been looked at before,'' Cox said.

Also, "if specific grunts identify a specific food, that is something we haven't seen before," Cox said.

Dinner Bell

Study co-author Zuberbuhler believes that the grunts serve a social function, since the chimps hardly ever make the noises when they are eating alone.

The grunts may be a call to dinner, Zuberbuhler says. "Chimps may find it genuinely unpleasant to eat without others doing the same."

The Jane Goodall Institue's Cox has observed different behavior among chimps, however. "I'm not sure that I agree with chimps not grunting when they're alone. When infants, they grunt when we offer them food, but maybe that's because we're considered parents," Cox said.

The possible dinner-bell grunts may be related to certain human vocalizations, Zuberbuhler said.

"We don't like to eat in the presence of others who are not eating," he said.

"In many cultures humans coordinate the timing of starting a meal, for example, with vocal cues such as 'bon appetit.'"

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