Blueprints of Language in Chimpanzee Interactions
– by Bas van Boekholt & Simone Pika. Photo credit by Bas van Boekholt
The complexity of human language has no counterpart in the animal kingdom, even though some basic building blocks can be found in animal communication. However, there is not “one” human language, with great variability in languages across the world and cultures. Despite this, the way humans use their language in day-to-day conversations shows remarkable similarities. Human conversations tend to consist of short flexible alternating turns with very little overlap or gaps. Additionally, when interacting, we use our gaze and body posture to make clear who or what is part of the conversation (and who/what not). This underlying infrastructure of interactions has been termed “turn-taking” and extends beyond normal conversations into sign language as well as in interactions devoid of any language, such as for example, joint actions. Moreover, from the moment infants start to interact with the world around them, their interactions display precursors to this infrastructure and parts of this infrastructure have been found in the interactions of other primate species.
This evidence led Stephen Levinson to propose the interaction engine hypothesis, which suggests that our capacity to socially interact, and involved skills (e.g., intentionality, multimodality, turn-taking), paved the way for language to evolve in the first place. Specifically turn-taking has been suggested to be a very ancient mechanism with its building blocks appearing across the primate lineage. However, to systematically compare interactions across and within species, a comparative framework is needed. Pika and colleagues recently developed such a framework splitting human turn-taking into four key elements: (A) flexibility of turn-taking organization, (B) who is taking the next turn, (C) when do response turns occur, and (D) what should the next turn do. Following this framework, studies have shown some turn-taking elements in the interactions of other primate species, but they often focused on specific contexts or modalities (for example just looking at vocalisations). Additionally, very little is known about the development of these elements in the first few years of life. These biases have given us a limited perspective of the infrastructure underlying primate interactions and its ontogeny.
In this study, we wanted to address this gap by investigating whether interactions between young chimpanzee infants and their mother, involve two crucial elements of human turn-taking: temporal relationships (when do response turns occur) and participation frameworks (who is taking the next turn). Additionally, we investigated one more characteristic describing the infrastructure of interactions: the relative use of signals and actions. While most human interactions, such as conversations, consist of signal-signal exchanges with relatively little use of actions, this might not hold for interactions of other primate species, which are generally of a more imperative nature with individuals trying to achieve a physical goal. To investigate if these three characteristics are consistent in describing the infrastructure of chimpanzee interactions, we also determined if they were influenced by demographic and interactional factors such as the age and sex of the infant, the interactant class (infant or mother) or the context in which the interaction took place.
To answer these research questions, we filmed and analysed 360 interactions in 17 mother-infant dyads (infants: 0-5 year) over four different contexts of chimpanzees of the Ngogo population living in their natural environment in the Kibale National Park, Uganda over a two-year period.
Taking all interactions together, mother and infant chimpanzees used slightly more signals (N=1943) than actions (N=1704). Infants produced more signals and actions than their mothers, but the signal/action ration stayed the same. Moreover, the signal/action ratio did not change over development but was influenced by context, where, in the food-sharing context, there were relatively more signals produced and in the joint-travel context there were relative more actions. Most interactions (~86%) contained both signals and actions with less then 1% of the interactions consisting exclusively of signals. These results showcase the importance of both signals and actions in chimpanzee communication, and that, opposed to humans, chimpanzees do not have signal-only “conversations”.
For the temporal relationships we found that most responses fall within a zero to two second time window, with a threshold of three seconds. The timing of these response times was independent of the age of the infant, and did not differ between the infant and the mother or whether the response was to a signal or action. However, context had an influence with faster response times characterizing the joint-travel context. As we used slightly different methodologies, we cannot directly compare these response times to human studies, but the time window coincides with what previous studies have shown in adult chimpanzees.
Finally, throughout all interactions both mother and infant chimpanzees make frequent use of directed gaze and body direction to create and maintain a participation framework. Infants showed higher frequencies of directed gaze and body direction compared to their mother which might have been caused by dependence of young chimpanzee infants on their mother leading to a smaller social environment.
Concluding, we found that chimpanzee mother-infant interactions were characterized by an equal distribution of actions and signals, response times averaging around one second, and the establishment and maintenance of participation frameworks through high frequencies of directed gaze and body direction. These characteristics seem to be independent of distinct demographic and interactional factors, except for the influence of context. With a high occurrence of actions, chimpanzees of our study population did not seem to engage in signal-signal ‘conversations’. This could mean that the precursor of human conversational turn-taking was a complex system composed of interactions containing both signals and actions, similarly to interactions of human adults and infants. Subsequently, humans might have specialized in signal-signal conversations, that appear later in development.
Additionally, these interactional characteristics appear early in development, barely changing with aging infants, highlighting the consistent and fundamental nature of this infrastructure of mother-infant interactions. This is in line with the interaction engine hypothesis which suggests that this interactional infrastructure, together with other skills such as intentionality preceded the evolution of human language. However, future studies are needed to uncover if this infrastructure is present and comparable in other populations, species and age-classes across the primate lineage. Taking this comparative approach will help us to unravel the way ancient humans socially interacted, and if this infrastructure formed the blueprint from which language evolved.
Read the original paper: van Boekholt, B., & Pika, S. (2025). Infrastructure of mother-infant interactions across development in chimpanzees (Pan troglodytes) in the wild. Evolution & Human Behavior, 46(2), 106671.
