"They do not speak, of course, if we take human language as a reference," warns evolutionary biologist Cédric Girard-Buttoz, lead author of the study published Monday in Nature communications biology.

But chimpanzees have, according to the study, an extraordinary ability to combine a reduced range of cries, "twelve simple vocalizations" according to the researcher, in at least 390 distinct sequences.

A sort of syntax, associating from two to ten cries, the meaning of which remains to be clarified.

With the key to the possibility that the animal has developed the ability to associate sounds in different ways to mean different things.

- the "hoo"

"We have some ideas about the meaning of certain sequences, one of which, for example, is exclusively linked to nesting, and which has no connection with the meaning of the calls taken in isolation", explains to AFP the CNRS researcher from the Institute. of Cognitive Sciences Marc Jeannerod.

The team, some of whose members are also affiliated with the German Max Planck Institute for Evolutionary Anthropology, worked on the basis of nearly 5,000 recordings made from 46 wild adult chimpanzees in the Ivorian reserve of the Taï National Park.

A doctoral student, Tatiana Bortolato, recorded more than 800 hours of vocalizations there for six months, which were then listed and classified with the help of human "coders".

A new method, and perhaps to be used with other primates, such as the bonobo or the orangutan.

The twelve types of vocalizations in chimpanzees include the very common "hoo", associated or not with a gasp, or even growling or barking.

They are well known to scholars -and "Planet of the Apes" fans-, and their meaning can vary with intensity.

"The + hou + linked to an alarm cry is stronger in tone than that which is linked to rest, softer", says Mr. Girard-Buttoz.

The "hou" linked to a gasp is used, for example, to identify individuals.

Some animals have a richer "vocabulary", species of monkeys having up to 38 types of calls.

"We can consider the vocal repertoire of chimpanzees, which is innate, as limited", remarks Mr. Girard-Buttoz, but the species, which has a very complex social life, may have found with the sequences a way of " generate new meanings that expand their ability to communicate.

The study showed that a third of the vocalizations were sequences combining several of the twelve calls, the whole range of which was used at one time or another.

It also established a notion of order, with certain cries always or almost always being in the same position in a "bigram", a sequence that associates two cries.

"For example the +hou+ is almost always in first position, which potentially indicates a recurring communication structure", notes the researcher.

- monkey syntax

Louder, some bigrams are reused, perhaps as keywords, in longer sequences, with five or six other calls, further proof of a stable structure of this simian syntax.

It remains to understand the relationship between the elements of these sequences, and in particular whether some are generative of a new meaning.

A young chimpanzee in the Living Museum shelter in Bujumbura, Burundi, on March 14, 2022 Yasuyoshi Chiba AFP

The researchers thus noticed variations between populations of chimpanzees on the order of the cries in certain sequences, and this “in a very consistent way”, says Mr. Girard-Buttoz.

This would imply a learning of these vocal sequences within the group, and therefore a flexibility of this mode of communication.

To go further, scientists will have to look for the meaning of the sequences, with playback experiments.

“We record a sound and play it as a monkey in the forest to study its reaction,” says the researcher.

The task will be daunting, given the number of sequences identified.

© 2022 AFP