Chimpanzees Asanti and Akuna vocalising. A new study shows that wild chimpanzees use a variety of call combinations to expand messaging. © Liran Samuni, Taï Chimpanzee Project.

Wild chimpanzees alter the meaning of single calls when embedding them into diverse call combinations, mirroring linguistic operations in human language.

Humans are the only species on earth known to use language. They do this by combining sounds into words and words into sentences, creating infinite meanings.

This process is based on linguistic rules that define how the meaning of calls is understood in different sentence structures.

For example, the word ‘ape’ can be combined with other words to form compositional sentences that add meaning: ‘the ape eats’ or append meaning: ‘big ape’, and non-compositional idiomatic sentences that create a completely new meaning: ‘go ape’.

A key component of language is syntax, which determines how the order of words affects meaning, for instance how ‘go ape’ and ‘ape goes’ convey different meanings.

One fundamental question in science is to understand where this extraordinary capacity for language originates from.

Researchers often use the comparative approach to trace the evolutionary origins of human language by comparing the vocal production of other animals, particularly primates, with that of humans.

Unlike humans, other primates typically rely on single calls (referred to as call types), and while some species combine calls, these combinations are only a few per species and mostly serve to alert others to the presence of predators.

This suggests that their communication systems may be too restricted to be a precursor to the complex, open-ended combinatorial system that is human language.

However, we may not have a full picture of the linguistic capacities of our closest living relatives, particularly how they might use call combinations to significantly expand their meaning.  

The researchers recorded thousands of vocalisations from three groups of wild chimpanzees in the Taï National Park in Ivory Coast. © Liran Samuni, Taï Chimpanzee Project.

Studying the meaning of chimpanzee vocalisations

Researchers from the Max Planck Institutes for Evolutionary Anthropology and for Cognitive and Brain Sciences in Leipzig, Germany, and from the Cognitive Neuroscience Center Marc Jeannerod (CNRS/Université Claude Bernard Lyon 1) and Neuroscience Research Center (CNRS/Inserm/Université Claude Bernard Lyon 1) in Lyon, France recorded thousands of vocalisations from three groups of wild chimpanzees in the Taï National Park in Ivory Coast.

They examined how the meanings of 12 different chimpanzee calls changed when they were combined into two-call combinations.

Senior author Catherine Crockford said: “Generating new or combined meanings by combining words is a hallmark of human language, and it is crucial to investigate whether a similar capacity exists in our closest living relatives, chimpanzees and bonobos, in order to decipher the origins of human language.”

Roman Wittig, co-author of the study and director of the Taï Chimpanzee Project, added: “Recording chimpanzee vocalisations over several years in their natural environment is essential in order to document their full communicative capabilities, a task that is becoming increasingly challenging due to growing human threats to wild chimpanzee populations.”

Chimpanzees’ complex communication system

The study reveals four ways in which chimpanzees alter meanings when combining single calls into 16 different two-call combinations, analogous to the key linguistic principles in human language.

Chimpanzees used compositional combinations that added meaning (eg, A = feeding, B = resting, AB = feeding + resting) and clarified meaning (eg, A = feeding or travelling, B = aggression, AB = travelling).

They also used non-compositional idiomatic combinations that created entirely new meanings (eg, A = resting, B = affiliation, AB = nesting).

Crucially, unlike previous studies which have mostly reported call combinations in limited situations such as predator encounters, the chimpanzees in this study expanded their meanings through the versatile combination of most of their single calls into a large diversity of call combinations used in a wide range of contexts.

First author Cédric Girard-Buttoz said: “Our findings suggest a highly generative vocal communication system, unprecedented in the animal kingdom, which echoes recent findings in bonobos suggesting that complex combinatorial capacities were already present in the common ancestor of humans and these two great ape species.

“This changes the views of the last century which considered communication in the great apes to be fixed and linked to emotional states, and therefore unable to tell us anything about the evolution of language.

“Instead, we see clear indications here that most call types in the repertoire can shift or combine their meaning when combined with other call types.”

He added: “The complexity of this system suggests either that there is indeed something special about hominid communication – that complex communication was already emerging in our last common ancestor, shared with our closest living relatives – or that we have underestimated the complexity of communication in other animals as well, which requires further study.”

The study is published in Science Advances.