The Ontogeny of Vocal Rhythms in a Non-Human Primate.

Rhythm is a fundamental aspect of human behaviour, and musical rhythm provides one of its most elaborate instances. Unlike speech, this rhythmic behaviour is characterized by the production of temporal patterns structured around small-integer ratios, which emerge early in life and change systematically across development. Whether such developmental trajectories are uniquely human or reflect broader biological constraints remains an open question. Here, we adopt a comparative developmental approach to map the ontogeny of rhythmic structure in the vocalizations of a non-human primate, the singing lemur Indri indri. We recorded songs from individuals of different age classes and quantified temporal organization by measuring inter-onset intervals between successive note onsets. From these intervals, we computed rhythmic ratios between adjacent units and assessed their correspondence to small-integer values. We find that isochrony (1:1 ratios), a core feature of human rhythm, is present from the earliest stages of vocal production. Over development, indris produce an increasing diversity of rhythmic structures corresponding to simple numerical relationships between adjacent intervals. This similarity to humans contrasts with three key differences. First, in indri, binary ratios (1:2 and 2:1) emerge gradually. Second, rhythmic precision around small-integer ratios does not systematically increase with age. Third, developmental trajectories differ between males and females. Together, these findings reveal both shared and divergent developmental pathways of rhythm production in humans and non-human primates, suggesting that early-emerging temporal regularity (i.e., isochrony) may reflect conserved biological constraints, whereas later-developing aspects of rhythmic structure are shaped by species-specific developmental processes.