Socially scripted vocal learning in primates

Generative vocal plasticity in chimpanzees

Modern theory posits that human-ape differences in voice command account for speech evolution. However, comparison has been indirect and conjectural based on vocal learning taxa far related from Hominids, instead of direct and quantitative based on great ape calls that, like all speech sounds, are local-specific and non-universal to the species. Moreover, the null hypothesis that the great ape voice command is purely reflexive has never been directly tested. Here, we show that in controlled, constant experimental settings, captive chimpanzees exhibit high-dimensional dexterity over voice activation and modulation in two atypical vowel-like calls. Subjects made unrestricted, multidimensional, and distinct voice changes within and between individuals, inducing parameter changes up to 10,000%, rejecting null hypothesis' predictions. Forecasting models indicated unmitigated voice novelty, altogether demonstrating emancipated and vast real-time voice control. Findings show that, contrary to traditional assumptions, speech and song evolution likely hinged on prolific voice command already available in ancestral ape-like ancestors.

Social and individual factors mediate chimpanzee vocal ontogeny

Human language develops in social interactions. In other ape species, the role of social learning in vocal ontogeny can be typically underappreciated, mainly because it has received little empirical attention. Here, we examine the development of pant hoot vocalisations during vocal exchanges in immature wild chimpanzees (Pan troglodytes schweinfurthii) of the Sonso community of the Budongo Forest, Uganda. We investigated how maternal gregariousness, age, sex, and social context are associated with behavioural and vocal responses to other group members' calls. We show that the older sons of gregarious mothers are more likely to orient their attention, respond vocally to the calls of others, and are overall more exposed to others' calls compared to other immature individuals. This effect is strongest in the presence of adult males and when their mothers also respond vocally, suggesting that chimpanzee vocal development is enhanced by social and vocal exposure. Our findings are consistent with a more flexible and socially mediated chimpanzee vocal ontogeny than previously assumed and show some parallels with animal vocal learners and children language acquisition.