Commonalities and group differences in the communicative efficiency of chimpanzee gesturing

Abstract

In animal communication research, there is a keen interest in assessing and comparing the efficiency of communication systems with human languages. Yet, the gestural communication of our closest relatives, chimpanzees, remains underexplored. This thesis addresses this gap by investigating communicative efficiency in gestural communication across three chimpanzee communities: Sonso and Waibira in East Africa (Uganda) and Bossou in West Africa (Guinea). Employing a novel coding scheme and diverse gesture duration metrics, I examine the presence of Zipf's law of brevity and Menzerath's law, commonly expressed principles of communicative efficiency in human language, in chimpanzee gesturing. In addition, I directly assess the optimality of chimpanzee gestures using a novel linguistic metric from human language comparisons. I explore variations in gesture durations and sequence usage based on individual and social characteristics, such as familiarity, gender, age, and rank. Across these measures, a clear pattern emerges: Each chimpanzee community exhibits a distinct communicative style closely tied to their socio-ecological context. Remarkably, gestures in Bossou, the most cohesive community, demonstrate the first confirmed presence of Zipf's law of brevity and Menzerath's law, along with gesture repertoire optimisation reaching the lower bound of the distribution represented in human languages. Conversely, Waibira, the largest but least cohesive community, employs gestures with redundancy, favouring short sequences and lengthy single gestures. Sonso falls between these extremes, both in community dynamics and communicative efficiency. A consistent finding is that, as familiarity increases, gesture durations also lengthen, suggesting closer social connections lead to reduced pressure to communicate quickly or efficiently. Ultimately, each community employs a different communication style, highlighting the flexible potential of chimpanzee gestures.