Human language is a fundamentally cooperative enterprise, embodying fast-paced and extended social interactions. It has been suggested that it evolved as part of a larger adaptation of humans’ species-unique forms of cooperation. Although our closest living relatives, bonobos and chimpanzees, show general cooperative abilities, their communicative interactions seem to lack the cooperative nature of human conversation. Here, we revisited this claim by conducting the first systematic comparison of communicative interactions in mother-infant dyads living in two different communities of bonobos (LuiKotale, DRC; Wamba, DRC) and chimpanzees (Taï South, Côte d’Ivoire; Kanyawara, Uganda) in the wild. Focusing on the communicative function of joint-travel-initiation, we applied parameters of conversation analysis to gestural exchanges between mothers and infants. Results showed that communicative exchanges in both species resemble cooperative turn-taking sequences in human conversation. While bonobos consistently addressed the recipient via gaze before signal initiation and used so-called overlapping responses, chimpanzees engaged in more extended negotiations, involving frequent response waiting and gestural sequences. Our results thus strengthen the hypothesis that interactional intelligence paved the way to the cooperative endeavour of human language and suggest that social matrices highly impact upon communication styles.

The evolution and function of human and nonhuman primate facial expression can be better understood through multispecies comparative analyses. Anatomically based coding systems (Facial Action Coding Systems [FACS]) are developed to enable such analyses because they are standardized and systematic and aid identification of homologous expressions underpinned by similar muscle contractions. To date, FACS has been developed for humans, and subsequently modified for chimpanzees and rhesus macaques. Through detailed examination of facial anatomy and spontaneous movement, here we present a FACS for hylobatids (gibbons and siamangs): apes living in small groups and often thought not to display extensive facial movement. We identified and described 18 independent facial movements (Action Units and Action Descriptors) in the hylobatid face, and compared to humans, chimpanzees, and rhesus macaques. The findings show that the facial movement capacity of hylobatids bears great similarity to species living in much larger groups. The findings could be explained by phylogenetic inertia (especially because pair-bonding is likely to have emerged from social living) and/or the social demands of pair-bonding.

Comparative studies in relation to language origins have mainly focused on our closest living relatives, bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Direct comparisons however are still lacking and/or concerned individuals living in captive environments only. Here, we carried out a systematic, quantitative comparison of communicative abilities of bonobos and chimpanzees living in four different communities in the wild with a special focus on within-and between-species variability. The analyses focused on communicative exchanges between mother-infant dyads to initiate joint travel. The communicative behavior of twelve bonobo (LuiKotale, Wamba; DRC) and thirteen chimpanzee mother-infant dyads (Taï South, Côte d'Ivoire; Kanyawara, Uganda) was filmed during a total of 1033 (bonobos) and 1189 (chimpanzees) hours of observation. We analyzed 'signal-response' structures (e.g. response waiting, gestural sequences, temporal relationships) in 316 bonobo and 415 chimpanzee carry initiations, while taking into account dyadic role, infant age and site. Differences in behavior could not be attributed to within-species variability. Bonobos solicited carries more frequently from shorter distances and via coordinated responses. Chimpanzees used a higher frequency of response waiting and sequences, showing overall more persistence. Our findings suggest that interactional intelligence paved the way to language with communication styles being influenced by social matrices.PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.1791v1 | CC-BY 4.0 Open Access |

Behavioural research on non-vocal communication among non-human primates and its possible links to the origin of human language is a long-standing research topic. Because human language is under voluntary control, it is of interest whether this is also true for any communicative signals of other species. It has been argued that the behaviour of hiding a facial expression with one's hand supports the idea that gestures might be under more voluntary control than facial expressions among non-human primates, and it has also been interpreted as a sign of intentionality. So far, the behaviour has only been reported twice, for single gorilla and chimpanzee individuals, both in captivity. Here, we report the first observation of concealing of facial expressions by a monkey, a Barbary macaque (Macaca sylvanus), living in the wild. On eight separate occasions between 2009 and 2011 an adult male was filmed concealing two different facial expressions associated with play and aggression ("play face" and "scream face"), 22 times in total. The videos were analysed in detail, including gaze direction, hand usage, duration, and individuals present. This male was the only individual in his group to manifest this behaviour, which always occurred in the presence of a dominant male. Several possible interpretations of the function of the behaviour are discussed. The observations in this study indicate that the gestural communication and cognitive abilities of monkeys warrant more research attention.

Comparative studies in relation to language origins have mainly focused on our closest living relatives, bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Direct comparisons however are still lacking and/or concerned individuals living in captive environments only. Here, we carried out a systematic, quantitative comparison of communicative abilities of bonobos and chimpanzees living in four different communities in the wild with a special focus on within-and between-species variability. The analyses focused on communicative exchanges between mother-infant dyads to initiate joint travel. The communicative behavior of twelve bonobo (LuiKotale, Wamba; DRC) and thirteen chimpanzee mother-infant dyads (Taï South, Côte d'Ivoire; Kanyawara, Uganda) was filmed during a total of 1033 (bonobos) and 1189 (chimpanzees) hours of observation. We analyzed 'signal-response' structures (e.g. response waiting, gestural sequences, temporal relationships) in 316 bonobo and 415 chimpanzee carry initiations, while taking into account dyadic role, infant age and site. Differences in behavior could not be attributed to within-species variability. Bonobos solicited carries more frequently from shorter distances and via coordinated responses. Chimpanzees used a higher frequency of response waiting and sequences, showing overall more persistence. Our findings suggest that interactional intelligence paved the way to language with communication styles being influenced by social matrices.PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.1791v1 | CC-BY 4.0 Open Access | rec