Ammie Kalan

"Kuba," an adult male chimp, photographed during a grooming session. Ammie Kalan

Researchers eavesdropping on wild chimpanzees determined that the primates communicate about at least two things: their favorite yummy fruits, and the trees where these fruits can be found.

Of particular interest to the chimps is the size of trees bearing the fruits that they relish most, such that the chimps yell out that information, according to a new study published in the journal Animal Behaviour.

The study is the first to find that information about tree size and available fruit amounts are included in chimp calls, in addition to assessments about food quality.

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"Chimpanzees definitely have a very complex communication system that includes a variety of vocalizations, but also facial expressions and gestures," project leader Ammie Kalan of the Max Planck Institute for Evolutionary Anthropology told Discovery News.

"How much it resembles human language is still a matter of debate," she added, "but at the very least, research shows that chimpanzees use vocalizations in a sophisticated manner, taking into account their social and environmental surroundings."

Kalan and colleagues Roger Mundry and Christophe Boesch spent over 750 hours observing chimps and analyzing their food calls in the Ivory Coast's Taï Forest. The Wild Chimpanzee Foundation in West Africa is working hard to try and protect this population of chimps, which is one of the last wild populations of our primate cousins.

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The researchers found that higher pitched calls were produced when the chimps encountered fruits from Nauclea trees. Smaller trees elicited still higher pitched calls, while calls associated with bigger trees with more fruit were lower in pitch.

In short, the chimps seem to "talk" a lot about Nauclea fruits!

"I never tried these fruits myself, but they do smell very good in the forest," Kalan said. "They are also quite big and easy to ingest, and we also know that they have a high energy content, which is important for wild animals."

Researcher Ammie Kalan recording chimp vocalizations in the Taï Forest.Ammie Kalan

A hallmark of human language is that vocalizations are “functionally referential.” Kalan and her team explained that this means the calls specifically refer to a particular individual, place, thing or happening, and that they must generate an appropriate response from those who hear the calls.

In this case, whenever the chimps called out after they found a large Nauclea fruit tree, their relatives would come running. The “dinner bell” effect is itself of interest, since it demonstrates how generous these primates can be.

"Chimpanzees are incredibly social beings, and sharing food is just one way of many that individual chimpanzees can solidify relationships within their group," Kalan said, adding that chimps travel in independent parties throughout the day, and almost never with their whole group at once.

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She continued, "Calling for one another at a food tree is one way that they can attract other chimpanzees in order to meet up with members of their group that they have not seen for a while."

Klaus Zuberbühler, a professor in the School of Psychology and Neuroscience at the University of St. Andrews, also studies primate communication. Zuberbühler told Discovery News, "What the authors have shown is that (chimp) call structure changes, not only in response to the food type, which was already known before, but also in relation to the size of the tree, a new finding."

He hopes that future research will investigate how much of the call information is meaningful for listening chimps. He suspects that they fully understand the calls, but this hasn’t been formally tested yet.