Infrastructure of mother-infant interactions across development in chimpanzees (Pan troglodytes) in the wild

The infrastructure underlying human social interaction can be described by several characteristics, such as the exchange of signals and actions, specific temporal relationships, and the use of directed gaze and body direction. These characteristics are remarkably uniform across several different languages, cultures with some of them emerging in mother-infant interactions early in development before the onset of words. It has been suggested that distinct features underlying human social action might have preceded the evolution of language and are shared across the whole primate lineage. However, despite decades of research on nonhuman primate communication, our understanding of general characteristics underlying communicative interactions, and, more specifically, the role they play in the development of communication, remains surprisingly limited. Hence, here we aimed to gain a more comprehensive overview, by studying mother-infant interactions of one of our closest living relatives, the chimpanzee (Pan troglodytes schweinfurthii), living in their natural environments. Specifically, we addressed the following two research questions: 1) Which characteristics built the main infrastructure of mother-infant interactions? 2) Which factors influence the infrastructure of mother-infant interactions? To answer these questions, we observed communicative interactions of a total of 17 chimpanzee mother-infant dyads (0–5 years) in the Ngogo community, Kibale National Park, Uganda between February 2021 and February 2023 (N = 1295 observation hours). We specifically focused on four different contexts where interactions frequently occurred, food sharing, nursing, grooming and joint-travel, and investigated the role of demographic factors (age and sex of the infant, interactant class) and interactional factors (context, unit type, turn transition). The results showed that mother-infant interactions were characterized by an equally distributed exchange of signals and actions, showed response times ranging from zero to two seconds, and involved the establishment and maintenance of participation frameworks through high frequencies of directed gaze and body direction. There was little to no effect of age and sex of the infant, interactant class, unit type and turn transition on these characteristics. However, context had a strong influence with relative lower frequencies of signals, quicker response times, and lower frequencies of directed gaze and body direction observed in the joint-travel context. By taking a comparative developmental approach, this study highlights commonalities in the infrastructure of mother-infant interactions between humans and chimpanzees, which contribute to uncover how extinct humans might have socially interacted.