What aspects of infants’ prelinguistic communication are most valuable for learning to speak, and why? We test whether early vocalizations and gestures drive the transition to word use because, in addition to indicating motoric readiness, they (a) are early instances of intentional communication and (b) elicit verbal responses from caregivers. In study 1, 11 month olds (N = 134) were observed to coordinate vocalizations and gestures with gaze to their caregiver's face at above chance rates, indicating that they are plausibly intentionally communicative. Study 2 tested whether those infant communicative acts that were gaze‐coordinated best predicted later expressive vocabulary. We report a novel procedure for predicting vocabulary via multi‐model inference over a comprehensive set of infant behaviours produced at 11 and 12 months (n = 58). This makes it possible to establish the relative predictive value of different behaviours that are hierarchically organized by level of granularity. Gaze‐coordinated vocalizations were the most valuable predictors of expressive vocabulary size up to 24 months. Study 3 established that caregivers were more likely to respond to gaze‐coordinated behaviours. Moreover, the dyadic combination of infant gaze‐coordinated vocalization and caregiver response was by far the best predictor of later vocabulary size. We conclude that practice with prelinguistic intentional communication facilitates the leap to symbol use. Learning is optimized when caregivers respond to intentional vocalizations with appropriate language.

Animals communicate using a vast array of different signals in different modalities. For chimpanzees, vocalizations, gestures and facial expressions are all important forms of communication, yet these signals have rarely been studied together holistically. The current study aimed to provide the first comprehensive repertoire of flexibly combined ('free') multimodal (MM) signals, and assess individual and contextual factors influencing production of, and responses to, unimodal (UM) and MM signals in wild chimpanzees. In total, 48 different free MM signals were produced. MM signals were produced at a significantly lower rate than UM signals, but 22 of 26 focal animals were observed to produce free MM signals. The relative production rates of different types of UM and MM signals differed significantly between the behavioural contexts investigated, showing flexible use of signals across contexts. In contrast, individual factors such as age, sex or rank of signaller did not appear to influence the type of signal produced or the likelihood of eliciting a response. Finally, we compared recipient responses to free MM grunt-gesture signals and matched UM component signals and found that these MM signals were more likely to elicit a response than a grunt alone, but were as likely to elicit a response as the gesture alone. The overall findings point to a widespread capacity for wild chimpanzees to flexibly combine signals from different modalities and highlight the importance of adopting a multimodal approach to studying communication.

Long-distance calls have a variety of functions in different animal species. However, where multiple functions are proposed for a single long-distance call type, little is known about their relative importance. Chimpanzees are one species where several functions have been proposed for their long-distance call, the pant hoot. In this study, we investigated the effect of social factors, including the rank of the caller, party size, fission–fusion rates, and the presence of estrus females as well as ecological factors including the type of food consumed and travel time, on male chimpanzee (Pan troglodytes schweinfurthii) pant hooting, in order to identify the key correlates of this behavior. The wild male chimpanzees of the Kanyawara community, Uganda, produced more pant hoots on days when there were frequent changes in the male, but not female, composition of the focal’s party. This factor accounted for the largest amount of variation in pant hoot production, and we found that males were more likely to repeat a call prior to rather than after fusion with other males, suggesting that the calls facilitate fusion. Pant hoots therefore seem to play a pivotal role in regulating grouping dynamics in chimpanzees. Our study also shows that pant hooting was positively correlated with the rank of the caller, the presence of parous females in estrus, and the consumption of high-quality food, suggesting that pant hoots signal social status or social bonds when between-male competition is high. This study supports the view that pant hoots fulfill a complex social function

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