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In most languages, individual words can be ambiguous between several different meanings, but through syntax and context the intended meaning of an ambiguous word usually becomes apparent. Many great ape gestures also have ambiguous meanings, which poses the problem of how individuals can interpret the signaller’s intended meaning in specific instances. We tested the effects of sequence compositionality and situational context (including behavioural and interpersonal contexts) in wild bonobos (Pan paniscus) at Wamba, DRCongo. We found no effect on a gesture’s meaning from its presence and position in sequence. However, two aspects of situational context did affect meaning: behaviour of the signaller immediately prior to gesturing, and relative age/sex of signaller and recipient. The intended meaning of ambiguous gestures was almost completely disambiguated by means of these aspects of context. Our findings suggest that the use of contextual information to interpret ambiguous signals predates the uniquely-human lineage and is not specific to language.

Animal communication has long been thought to be subject to pressures and constraints associated with social relationships. However, our understanding of how the nature and quality of social relationships relates to the use and evolution of communication is limited by a lack of directly comparable methods across multiple levels of analysis. Here, we analysed observational data from 111 wild groups belonging to 26 non-human primate species, to test how vocal communication relates to dominance style (the strictness with which a dominance hierarchy is enforced, ranging from 'despotic' to 'tolerant'). At the individual-level, we found that dominant individuals who were more tolerant vocalized at a higher rate than their despotic counterparts. This indicates that tolerance within a relationship may place pressure on the dominant partner to communicate more during social interactions. At the species-level, however, despotic species exhibited a larger repertoire of hierarchy-related vocalizations than their tolerant counterparts. Findings suggest primate signals are used and evolve in tandem with the nature of interactions that characterize individuals' social relationships. Publisher PDF

Cross-species comparison of great ape gesturing has so far been limited to the physical form of gestures in the repertoire, without questioning whether gestures share the same meanings. Researchers have recently catalogued the meanings of chimpanzee gestures, but little is known about the gesture meanings of our other closest living relative, the bonobo. The bonobo gestural repertoire overlaps by approximately 90% with that of the chimpanzee, but such overlap might not extend to meanings. Here, we first determine the meanings of bonobo gestures by analysing the outcomes of gesturing that apparently satisfy the signaller. Around half of bonobo gestures have a single meaning, while half are more ambiguous. Moreover, all but 1 gesture type have distinct meanings, achieving a different distribution of intended meanings to the average distribution for all gesture types. We then employ a randomisation procedure in a novel way to test the likelihood that the observed between-species overlap in the assignment of meanings to gestures would arise by chance under a set of different constraints. We compare a matrix of the meanings of bonobo gestures with a matrix for those of chimpanzees against 10,000 randomised iterations of matrices constrained to the original data at 4 different levels. We find that the similarity between the 2 species is much greater than would be expected by chance. Bonobos and chimpanzees share not only the physical form of the gestures but also many gesture meanings., Author summary Bonobos and chimpanzees are closely related members of the great ape family, and both species use gestures to communicate. We are able to deduce the meaning of great ape gestures by looking at the ‘Apparently Satisfactory Outcome’ (ASO), which reflects how the recipient of the gesture reacts and whether their reaction satisfies the signaller; satisfaction is shown by the signaller ceasing to produce more gestures. Here, we use ASOs to define the meaning of bonobo gestures, most of which are used to start or stop social interactions such as grooming, travelling, or sex. We then compare the meanings of bonobo gestures with those of chimpanzees and find that many of the gestures share the same meanings. Bonobos and chimpanzees could, in principle, understand one another’s gestures; however, more research is necessary to determine how such gestures and gesture meanings are acquired.

Research on great ape gestural communication has hitherto prioritised the signaller, focusing on the expressed repertoire, the set of gesture types that an individual deploys. In this study of wild bonobos (Pan paniscus), we also examine the understood repertoire, the set of gesture types that a recipient understands: a recipient understands a gesture if they respond to the signaller with an Apparently Satisfactory Outcome (ASO). Combining the understood and expressed repertoires substantially expands an individual’s overall repertoire. Hence, we propose that including the understood repertoire better represents an individual’s actual communicative repertoire, and may for the first time reveal gesture types received exclusively by specific subsets of individuals. Significant differences between expressed and understood repertoires might result from two things: (1) gestures are ritualised between signallers and recipients and may therefore be unidirectional in some cases, or (2) the circumstances of use differ among age-sex groups, limiting the need for an individual to use a certain gesture type. In the latter, we expect consistency in particular age-sex groups whereas unidirectional comprehension as a result of ritualization would predict random distribution among individuals. We further compare our dataset with and without gesture instances occurring in play, as play may be a context in which individuals deploy gestures that they rarely or never use otherwise. Assessing the understood repertoire is a useful tool for animal communication researchers, not only allowing shorter study-periods, but also to discover gesture types specific to subsets of individuals, which may reveal much about a species’ social behaviour.

Great apes give gestures deliberately and voluntarily, in order to influence particular target audiences, whose direction of attention they take into account when choosing which type of gesture to use. These facts make the study of ape gesture directly relevant to understanding the evolutionary precursors of human language; here we present an assessment of ape gesture from that perspective, focusing on the work of the "St Andrews Group" of researchers. Intended meanings of ape gestures are relatively few and simple. As with human words, ape gestures often have several distinct meanings, which are effectively disambiguated by behavioural context. Compared to the signalling of most other animals, great ape gestural repertoires are large. Because of this, and the relatively small number of intended meanings they achieve, ape gestures are redundant, with extensive overlaps in meaning. The great majority of gestures are innate, in the sense that the species' biological inheritance includes the potential to develop each gestural form and use it for a specific range of purposes. Moreover, the phylogenetic origin of many gestures is relatively old, since gestures are extensively shared between different genera in the great ape family. Acquisition of an adult repertoire is a process of first exploring the innate species potential for many gestures and then gradual restriction to a final (active) repertoire that is much smaller. No evidence of syntactic structure has yet been detected.

Funding was provided by the Wenner-Gren Foundation Dissertation Fieldwork Grant, Japan Society for the Promotion of Science (JSPS) Grant in Aid for Scientific Research, JSPS Core-to-Core Program, and University of St Andrews 600th Anniversary Scholarship. In animal communication, signallers and recipients are typically different: each signal is given by one subset of individuals (members of the same age, sex, or social rank) and directed towards another. However, there is scope for signaller-recipient interchangeability in systems where most signals are potentially relevant for all age-sex groups, such as great ape gestural communication. In this study of wild bonobos (Pan paniscus), we aimed to discover whether their gestural communication is indeed a mutually understood communicative repertoire, in which all individuals can act as both signallers and recipients. While past studies have only examined the expressed repertoire, the set of gesture types that a signaller deploys, we also examined the understood repertoire, the set of gestures to which a recipient reacts in a way that satisfies the signaller. We found that most of the gestural repertoire was both expressed and understood by all age and sex groups, with few exceptions, suggesting that during their lifetimes all individuals may use and understand all gesture types. Indeed, as the number of overall gesture instances increased, so did the proportion of individuals estimated to both express and understand a gesture type. We compared the community repertoire of bonobos to that of chimpanzees, finding an 88% overlap. Observed differences are consistent with sampling effects generated by the species’ different social systems, and it is thus possible that the repertoire of gesture types available to Pan is determined biologically. Publisher PDF