Chimpanzees learn when to use some of their calls - with parallels to how humans learn words

To the Point

• Rethinking chimpanzee calls: It has long been assumed that wild chimpanzee vocalisations are entirely inherited and unlearned. A new study now challenges this view by revealing the first evidence of arbitrary, learned vocal conventions in chimpanzees.
• A "raspberry" call with flexible usage: Researchers compared an unvoiced call, similar to a 'raspberry' sound, in wild chimpanzees in Taï National Park, Côte d’Ivoire, and in orphaned, peer-raised chimpanzees at the Tacugama Chimpanzee Sanctuary, Sierra Leone.
• Learned, not innate: Wild chimpanzees used the call almost exclusively during grooming, whereas the sanctuary orphans used it during a wide range of social interactions, which suggests that the function of the call is learnt rather than biologically fixed.
• Learning from adults: When an adult was introduced to a group of orphaned juveniles, the group gradually began to use the call in a way that matched the adult's pattern. This suggests that young chimpanzees learn the functions of calls from older role models, just as humans do.

A new study led by researchers at the CNRS Institute of Cognitive Sciences in Lyon and the Taï Chimpanzee Project in the Ivory Coast has found the first evidence of arbitrary, conventionalised vocal communication in wild chimpanzees. This discovery narrows the perceived gap between chimpanzee and human communication.

The words of human languages are, for the most part, both learnt and arbitrary. To say that words are arbitrary means that, in a slightly different world, they could have quite different meanings. These arbitrary uses of words are known as linguistic conventions and vary across communities. We follow these conventions only because our peers do, and if they changed their usage, we would too. If English speakers had started using the German word 'Hund' or the French word 'chien' instead of 'dog' at some point in history, those words could easily have taken hold. As Shakespeare put it in Romeo and Juliet (Act II, Scene II): 'What's in a name? That which we call a rose / By any other name would smell as sweet.'

Until now, it was thought that the vocalisations of wild chimpanzees lack this kind of flexibility — that they are part of a fixed, species-wide biological inheritance. If vocalisations are not learnt, they cannot be arbitrary, since their users could not have chosen to use the same calls for different purposes. However, this new study challenges both of these assumptions, at least with regard to part of the chimpanzee vocal repertoire.

Comparing wild and orphaned chimpanzees

Doctoral student Antoine Valet and a team led by Catherine Crockford studied the use of an unvoiced chimpanzee call that resembles the sound of someone blowing a raspberry. This call is known to be part of the repertoire of both wild and captive chimpanzees. They compared its use among 91 chimpanzees, ranging in age from 0 to 55 years, in four wild communities in Taï National Park, Côte d’Ivoire, and four groups of orphaned, peer-raised chimpanzees at Tacugama Chimpanzee Sanctuary, Sierra Leone.

The results were striking: wild Taï chimpanzees used the call almost exclusively while grooming their peers, whereas the orphaned juveniles at Tacugama used it chaotically across quite different social interactions. This shows that the use of this call is not, as was previously believed, unlearned. If it was, all groups would use the call in the same way. Since they do not, the grooming function of the raspberry call must be learnt.

Learning from adult role models

A second part of the study examined what happened when an adult chimpanzee was introduced to one of the orphan groups at Tacugama. Over time, the group's use of the raspberry call became markedly more uniform, converging on the pattern used by the newly integrated adult. This suggests that this call is not only learned, but also specifically learned from adult role models within the group. “This means there's more continuity between chimpanzees' vocal learning and our own than we previously suspected,” says Richard Moore, a University of Warwick-based philosopher and co-author of the study.

If chimpanzees learn the raspberry call's function from older peers, one question remains: why do all wild chimpanzee populations appear to use the call in exactly the same way, rather than exhibiting the kind of variation observed in human languages? One possibility is that the raspberry call has an additional non-communicative origin, making it especially likely to be adopted during grooming. For example, the oral movements used to remove parasites from fur, such as lip protrusion or blowing air to part the fur, could easily evolve into a communicative signal simply by adding sound. Different chimpanzee groups may have discovered this useful feature independently, converging on a common pattern of use over time. However, this would not undermine the conventional nature of the call, since groups could have equally settled on different ways of signaling. Further research is needed to confirm or rule out this explanation.

Recent research has also observed variation in how chimpanzees construct call combinations across populations, suggesting that the true extent of variation and learning in chimpanzee vocal communication is not yet fully understood. “Even aspects of chimpanzee vocal communication that seemed fixed and unlearned may be shaped by learning, suggesting that there is still much to discover about their vocal repertoire,” said Antoine Valet, first author of the study.

A closer parallel to human communication

Taken together, the findings suggest that chimpanzee vocal communication is more similar to human communication than previously thought. Unlike humans, chimpanzees do appear to struggle to learn entirely new call sounds, suggesting that parts of their vocal repertoire are likely innate. However, in an intriguing parallel with humans, they seem to learn at least some of the functions of their calls from older members of their community. “Recent research strongly suggests that they may even learn to combine some calls into sequences,” says Cédric Girard-Buttoz, a senior author on the study.

Questions remain about how much learning shapes chimpanzee vocal communication overall and how this compares with other species. “Preserving wild chimpanzee populations is critical if we want to understand the extent to which they learn and perpetuate their vocal communication,” says Roman Wittig, director of the Taï Chimpanzee Project and a senior author of the study.