"We don't know yet how specific these calls arei.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.
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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