Coupled Oscillator Dynamics of Vocal Turn-Taking in Monkeys

Cooperation is central to human communication [1-3]. The foundation of cooperative verbal communication is taking turns to speak. Given the universality of turn-taking [4], it is natural to ask how it evolved. We used marmoset monkeys to explore whether another primate species exhibits cooperative vocal communication by taking turns. Marmosets share with humans a cooperative breeding strategy and volubility. Cooperative care behaviors are thought to scaffold prosocial cognitive processes [5,6]. Moreover, marmosets and other callitrichid primates are very vocal and readily exchange vocalizations with conspecifics [7-11]. By measuring the natural statistics of marmoset vocal exchanges, we observed that they take turns in extended sequences and show that this vocal turn-taking has as its foundation dynamics characteristic of coupled oscillators one that is similar to the dynamics proposed for human conversational turn-taking [12]. As marmoset monkeys are on a different branch of the evolutionary tree that led to humans, our data demonstrate convergent evolution of vocal cooperation. Perhaps more importantly, our data offer a plausible alternative scenario to "gestural origin" hypotheses for how human cooperative vocal communication could have evolved.