Coupled oscillator dynamics of vocal turn-taking in monkeys.

Cooperation is central to human communication. The foundation of cooperative verbal communication is taking turns to speak. Given the universality of turn-taking, 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. Moreover, marmosets and other callitrichid primates are very vocal and readily exchange vocalizations with conspecifics. 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. 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.