Convergent evolution of vocal cooperation without convergent evolution of brain size.

One pragmatic underlying successful vocal communication is the ability to take turns. Taking turns - a form of cooperation - facilitates the transmission of signals by reducing the amount of their overlap. This allows vocalizations to be better heard. Until recently, non-human primates were not thought of as particularly cooperative, especially in the vocal domain. We recently demonstrated that common marmosets (Callithrix jacchus), a small New World primate species, take turns when they exchange vocalizations with both related and unrelated conspecifics. As the common marmoset is distantly related to humans (and there is no documented evidence that Old World primates exhibit vocal turn taking), we argue that this ability arose as an instance of convergent evolution, and is part of a suite of prosocial behavioral tendencies. Such behaviors seem to be, at least in part, the outcome of the cooperative breeding strategy adopted by both humans and marmosets. Importantly, this suite of shared behaviors occurs without correspondence in encephalization. Marmoset vocal turn taking demonstrates that a large brain size and complex cognitive machinery is not needed for vocal cooperation to occur. Consistent with this idea, the temporal structure of marmoset vocal exchanges can be described in terms of coupled oscillator dynamics, similar to quantitative descriptions of human conversations. We propose a simple neural circuit mechanism that may account for these dynamics and, at its core, involves vocalization-induced reductions of arousal. Such a mechanism may underlie the evolution of vocal turn taking in both marmoset monkeys and humans.