Vocal sequences emerge slowly in chimpanzee development

Tatiana Bortolato, Roman Wittig, Cédric Girard-Buttoz and Catherine Crockford

The study of vocal patterns in our closest living relatives might be crucial to understand the building blocks of human language. A unique feature of language is its capacity to recombine a limited sound set into words, which are flexibly and hierarchically combined to create endlessly new meanings. However, non-human primate species generally use very few vocal sequences, although the sound set (vocal repertoire) is comparable in size to that of humans. When reported, vocal sequences occur in long-distance calls or alarm calls, such that the potential for meaning expansion through combination seems limited. However, a recent chimpanzee study shows the use of numerous different vocal sequences, across the whole vocal repertoire, including both loud and quiet calls, with positional and transitional bias in call order. This suggests a potential for a highly flexible combinatorial system. But how does such a flexible system develop? When in ontogeny is the adult combinatorial capacity acquired? To address these questions, we recorded 10.929 vocal utterances from 98 wild chimpanzees living in three different communities aged from 0 to 55 years old in the Taï National Park, Ivory Coast. We examined three different structural components of vocal sequence complexity: 1) the maximum length of the utterance (i.e. the number of different types of vocal units contained in an utterance, involving articulatory coordination), 2) the probability of an utterance to include panted units (calls with phonation during both inhalation and exhalation, involving respiratory coordination) and 3) the repertoire diversity (i.e. the number of different vocal utterances). We found that each structural component of vocal sequence complexity followed a clear developmental trajectory. Chimpanzees produced few, short and simple utterances in the first year of life. There was a clear increase in the length, diversity and probability to produce panted utterances throughout ontogeny. The steepest increase occurred around the weaning age and adult levels were reached at the beginning of the sub-adult life. We found neither sex nor rank differences in the ontogenetic trajectories. In this study, we provide the first systematic analysis of the development of structural complexity of vocal sequences and combinatorial potential in chimpanzees’ development and discuss potential maturational processes that may account for this developmental progression.