Convergence and Divergence: Forces Shaping Marmoset Vocal and Social Interactions

Abstract

Human language hinges on a remarkable capacity for vocal production learning, yet the evolutionary origins of this ability remain poorly understood. This thesis explores the idea that increasing cooperation and social interdependence played a major role in the evolution of vocal flexibility in the hominin lineage. To investigate this possibility, I studied common marmosets (Callithrix jacchus), a primate that shares several socio-communicative traits with humans, including cooperative breeding, proactive prosociality, vocal turn-taking, infant babbling and vocal convergence. The thesis begins by examining how learning emerges through social interactions. A review of the literature highlights the importance of contingent social feedback in shaping development, learning, and coordination. In cooperative breeding systems, interactions with multiple caregivers create rich opportunities for social learning and may provide conditions that favor the evolution of flexible communication. The first empirical study investigated vocal accommodation during pair formation as a process of vocal convergence. Newly paired marmosets gradually modified their vocalizations to become more similar to one another. Mathematical modeling suggested that this process is driven by continuous auditory-guided adjustments based on partner vocalizations. These findings indicate that vocal flexibility in marmosets is an active, socially mediated process, providing insight into mechanisms that may represent evolutionary precursors of more advanced vocal production learning. The second empirical study examined vigilance coordination during feeding. Marmosets coordinated these behaviors through anti-phase synchronization and convergence of rhythms. This coordination resulted in longer uninterrupted feeding bouts and emerged through continuous adjustments to partner behavior. Similar interactional dynamics were observed in both vocal accommodation and vigilance coordination, suggesting that communication and cooperation rely on common mechanisms of social synchrony. The third empirical study examined the role of vocal divergence in social boundary formation. While individuals vocally converged within groups, neighboring groups actively diverged from one another, demonstrating that the same vocal flexibility underlying convergence can also generate divergence, contributing to both within-group social cohesion and between-group boundary maintenance. The results parallel processes underlying dialect formation in humans and provide evidence that socially driven vocal differentiation is not unique to our species. The final empirical chapter examined whether vocal convergence in marmosets is socially regulated and whether individuals hold strong expectations about how social partners should vocalize during interactions. Within an interactive closed-loop playback paradigm, marmosets engaged in natural vocal exchanges with a speaker wherein playbacks were generated to either converge toward or diverge away from the focal's calls. Marmosets responded positively to converging calls and negatively to diverging calls, and they actively modified their own vocalizations to re-establish similarity following divergence from the speaker. These findings suggest that vocal convergence carries social significance and may function as a socially regulated norm. Taken together, the findings show that convergence and divergence are fundamental forces shaping marmoset vocal and social behavior. Vocal convergence, behavioral synchrony, vocal divergence, and social reinforcement come out as interconnected aspects of a communication system shaped by life in highly interdependent social groups. More generally, the results support the view that vocal production learning evolved gradually from ancestral forms of vocal plasticity already present in primates. Cooperative social systems may have provided the selective pressures under which increasingly flexible forms of vocal behavior became advantageous. Vocal accommodation offers a plausible starting point for this process because it already involves auditory monitoring, vocal modification, and social feedback. Overall, this thesis argues that cooperation and communication evolved in close association. The findings support the hypothesis that increasing social interdependence, cooperative breeding, and contingent social interactions were important factors in the evolutionary pathway leading from ancestral primate vocal flexibility to the full-fledged vocal production learning that underlies human language.