How do bonobos feel about their food? To find out, just listen to what they say when presented with a desirable or disgusting morsel, suggests a new study.
Bonobos yell out their food ratings using at least five distinct vocalizations, the study found. Since the calls are tonally similar to certain other primate sounds, such as the human exclamations "Yum!" and "Ewww," the scientists think there might be a somewhat universal primate language when it comes to food.
"It's an interesting possibility, which seems to be true for apes and humans," co-author Klaus Zuberbuhler told Discovery News, explaining that both groups can "modify the acoustic structure of their calls, and so reflect the quality of the food they've encountered."
Zuberbuhler, a professor in the School of Psychology at the University of St. Andrews in Scotland, and colleague Zanna Clay began their experiments by documenting the food preferences of bonobos housed at the San Diego Zoo and the San Diego Wild Animal Park. The great apes were presented with foods on a tray, and the scientists noted their first choices.
Figs got a perfect score, with raisins a close second. In order from most to least favored, the rest of the list read: grapes, bananas, popcorn, apples, oranges, biscuits, celery, melon, lettuce, yams and peppers.
"It appears that energy-rich succulent fruits were the most favored of foods," Clay told Discovery News. "A bit like us, they weren't so keen on their vegetables."
The scientists then recorded the bonobos' vocalizations as they interacted with each type of food. Specialized software identified the precise structure of the calls.
The researchers concluded that bonobos produce five distinct types of calls in response to food: barks, peeps, peep-yelps, yelps and grunts. These are documented in a paper accepted for publication in the journal Animal Behavior.
"Bonobo vocalizations are surprisingly high-pitched, and their food calls sound more like the shrill calls of young children or birds rather than from a large ape," said Clay.
When presented with their favorite foods, the bonobos almost always barked. They grunted when encountering their least favorites. The other calls seemed to signify ratings in between, with peep-yelps falling in the middle range for nibbles the individual thought were so-so.
"Finding food is one of the most important challenges to any wild animal, and thus any signal indicating the discovery of food may provide useful information to receivers," Clay explained. "If variation in the vocal sequences provides information about food quality, receivers may be able to use this to decide whether to abandon a current activity or not."
The scientists suspect bonobos combine calls to produce more complex sequences, but she said, "Our understanding of bonobo vocalizations is in its infancy and this study is just tapping into a vocal system that is not yet fully understood."
The linguistic abilities of non-human primates have been debated for decades, with some researchers hesitant to think that these animals, and others, meaningfully transmit specific information to each other using sophisticated calls.
"The resistance to this in the scientific community is enormous," said William Fields, director of bonobo research at Iowa's Great Ape Trust.