Norm-like vocal behavior in a nonhuman primate

Human interlocutors have strong expectations that guide when, how, and what is communicated, but the evolutionary origins of such normative expectations remain poorly understood. Vocal accommodation in marmosets, where individuals modify their calls to match partners, offers a potential model. Within an interactive closed-loop playback paradigm, marmosets experienced partner calls that either converged toward or diverged away from their own call structure. Marmosets responded affiliatively to convergence, whereas diverging calls elicited protest vocalizations. Importantly, the protests stopped when the calls converged again. Effects were partner-specific, indicating that marmosets expect bond partners, but not strangers, to converge during a conversation. Furthermore, individuals modified their calls to restore similarity to dissimilar partner playbacks. These findings identify marmoset vocal accommodation as a socially regulated behavior with normative properties.