Beyond the harem: vocal complexity in a captive gelada (Theropithecus gelada) bachelor group

The social complexity hypothesis for communication posits that species with elaborate social systems require more sophisticated vocal communication to navigate social life. Geladas (Theropithecus gelada), who live in complex multilevel societies and form diverse social bonds, are ideal for testing this hypothesis. Males produce species-specific, or derived, calls that reflect social status and function in facilitating male-female bonding. However, although derived call use also occurs in male-male social interactions, their function in male-male communication remains unexamined. In this pilot study we investigate the use of derived calls within a captive all-male bachelor group and test two hypotheses: (1) derived calls signal bachelor dominance status, and (2) derived calls function to maintain male-male social bonds. Despite a limited sample size, our results show that bachelor leaders produce higher proportions of derived calls than bachelor followers, mirroring patterns found in harem groups. Furthermore, bachelor followers were most likely to use derived calls in affiliative and resting contexts, suggesting a role in social bonding. In contrast, bachelor leaders used them more broadly. They additionally produced derived calls during agonism and travel, thereby possibly reflecting their more elaborate leadership duties. These findings provide the first preliminary evidence that derived calls serve functions beyond male-female communication and may be integral to both status signalling and affiliative interactions among bachelor males. Our results on the vocal dynamics in male geladas shed further light on the co-evolution of social and vocal complexity and offer broader implications for the origins of human language.