Context-dependent rhythmicity in chimpanzee displays

Rhythm is an important component of human language and music production. Rhythms such as isochrony (intervals spaced equally in time) are also present in vocalizations of certain non-human species, including several birds and mammals. This study aimed to identify rhythmic patterns with music-based methods within the display behaviour of chimpanzees (Pan troglodytes), humans' closest living relatives. Behavioural observations were conducted on individuals from two zoo-housed colonies. We found isochronous rhythms in vocal (e.g. pants, grunts and hoots) as well as in motoric (e.g. swaying and stomping) behavioural sequences. Among individuals, variation was found in the duration between onsets of behavioural elements, resulting in individual-specific tempi. Despite this variation in individual tempi, display sequences were consistently structured with stable, isochronous rhythms. Overall, directed displays targeted at specific individuals were less isochronous than undirected displays. The presence of rhythmic patterns across two independent colonies of chimpanzees suggests that underlying mechanisms for rhythm production may be shared between humans and non-human primates. This shared mechanism indicates that the cognitive requirements for rhythm production potentially preceded human music and language evolution.