Isochrony in titi monkeys duets: social context as a proximate cause of duets’ rhythm and regularity

Music and rhythm are typical features of all human cultures, but their biological origins remain unclear. Recent investigations suggest that rhythmic features of human music are shared with animal vocalizations. Moreover, arousal is known to influence the structure of both human speech and animal sounds. We investigated coppery titi monkeys’ (Plecturocebus cupreus) duet rhythms to assess adherence to rhythmic patterns previously observed only in Old World primates and to deepen our understanding of the proximate causes of non-human primate song rhythm. Titis’ songs were remarkably isochronous, but their tempo depended on the social context: songs sung during territorial confrontations have a slower pace than during early morning singing. Songs had a faster tempo and were less regular when infants were present, suggesting a speed-accuracy trade-off. Finally, we found that pair-mates perform isochronous songs with the same precision, suggesting that isochrony plays a role in boosting pair coordination, as it does in other singing primates. Our investigation sheds light on the ultimate and proximate causes of primates’ isochronous rhythm, to our knowledge confirming its presence for the first time in a New World monkey and highlighting the role of social factors in shaping its timing and regularity in the short term.