The patterning and ontogeny of chimpanzee vocal sequences and brain pathways

Catherine Crockford, Cedric Girard-Buttoz, Emiliano Zaccarella, Tatiana Bortolato, Cornelius Eichner, Alfred Anwander, Carsten Jaeger, Tobias Graessle, Pawel Fedurek, Liran Samuni, Kamilla Pleh, Roman Wittig, Nik Weiskopf, Angela Friederici

A striking feature of the human species is our large brain, enabling some complex skills that surpass those of other species, such as tool use, social cognition, and in particular, language. The question of how the brain pathways supporting these skills evolved across phylogeny is still open, mainly due to a lack of studies directly comparing human and non-human primate brains and the related behavior. In a consortium, the Evolution of Brain Connectivity Project, we first examine the structure of vocal sequence and tool use by wild chimpanzees (Tai Chimpanzee Project, Ivory Coast), in adults and through ontogeny, and then examine a developmental sample of post mortem wild chimpanzee brains sourced after natural death.

Across 46 adults and including around 5,000 utterances, we found that the vocal sequences show positional and transitional properties, suggesting that permutation relationships exist between the calls. Vocal sequences were used extensively across the repertoire such that their flexibility exceeds that reported for old world monkeys, with implications for predictions for target regions of white-matter tracts across species. Using 10,000 vocalisations from 99 chimpanzees throughout chimpanzee ontogeny, we found that ordered call sequences – and tool use, such as nut cracking – emerged after two years of age and continued to increase in complexity until around 8 years of age, with implications for the development of relevant brain pathways.

When examining chimpanzee brain pathways, we focused on the dorsal tract, a white matter tract that is crucial for language in humans. Our preliminary tractography results suggest a possible emergence of a dorsal connection to the inferior frontal lobe between 2 and 4 years of age. Similar to brain development in humans, the chimpanzee data of different age groups indicate a strengthening of the dorsal tract connecting the inferior frontal lobe with the temporal/parietal regions. We point out similarities and differences in the dorsal tract across chimpanzees and humans and discuss the implications of these findings for the evolution of language.