Vocal sequence diversity and length remain stable across ontogeny in a catarrhine monkey (Cercocebus atys)

During childhood, human speech utterances increase steadily in complexity, length and diversity. In contrast, the vocal repertoire of non-human primates has long been considered fixed from birth. Recent studies showing the acquisition of vocal sequences during ontogeny in chimpanzees and marmosets challenge this view. Here we further explore the potential flexibility of non-human primate vocal production by comparing the vocal sequence repertoire across age groups in sooty mangabeys, a species with a rich sequence repertoire for a catarrhine monkey. We recorded 1844 utterances from 75 individuals from two wild groups in Taï National Park, Ivory Coast. We used custom-made Bayesian models specifically designed to estimate the individual repertoire size of vocal sequences while accounting for under-sampling of certain vocalisations in certain individuals. We hereby provide a tool to estimate vocal repertoire size applicable to other taxa. We found no relevant ontogenetic changes in vocal repertoire size and utterance length. Ontogenetic vocal sequence expansion is therefore not universal among primates that routinely use vocal sequences to communicate. Rather, this feature may have evolved independently in distantly-related taxa due to social features thought to promote vocal complexity, such as the complex social organisation of chimpanzees and the cooperative breeding systems of marmosets.