Multimodal signalling in wild and captive orangutans: redundancy versus complementarity
Evolution, Anthropology
Final Report Abstract
Arguably the most powerful social tool in the animal kingdom, human language is often narrowed down to speech. However, language is in fact an intrinsically multimodal phenomenon, as speech acts are tightly interlinked with visual signals across cultures, ages and tasks. Despite growing evidence that the communication of nonhuman primates, the main model for the evolution of language, is also intrinsically multimodal, most studies on primate communication have focused on gesture or vocalization only. Thus, the function of multimodal signals or signal combinations remains little understood. One central aim of this project was to test established hypotheses for complex signal function (redundancy vs refinement) for the first time in great apes, and to examine to what extent social opportunities and the ecological environment affect communicative interactions. Specifically, I investigated the production of and responses to unimodal and multimodal signals in two species of orangutans (Pongo abelii, P. pygmaeus) in captivity and in the wild, allowing for the comparison between highly different social and ecological settings. To that end, I obtained a large comparative sample (N > 8000 communicative acts) of wild and zoo-housed orang-utans of two specie, and assessed wild-captive contrasts through four different studies of communicative plasticity, focusing on (1) multisensory and multi-component communication, (2) non-vocal repertoires (i.e. sets of gesture types and facial expressions), (3) gestural redoings (i.e. gestural repetition and elaboration after communicative failure), and (4) individual variation in communicative repertoires and tactics. First, we found that signal components were used in composite rather than single ways when the presumed goal did not match the dominant interaction outcome, while multisensory acts were overall more effective than the respective unisensory acts. We concluded that multisensory acts primarily facilitate effectiveness, whereas multicomponent acts are relevant when interaction outcomes are less predictable. Second, we found that repertoires on both the individual and population level are larger in captive than wild settings, regardless of species, age class or sampling effort. In the more sociable Sumatran species, dominant use of signals towards single as opposed to multiple outcomes (i.e. functional specificity) was also higher in captive settings. Third, we examined gestural redoings and identified wild-captive contrasts in Borneans, but not in Sumatrans. Moreover, we found that the effectiveness of elaboration in eliciting responses was higher in Sumatrans, especially the captive ones, whereas effectiveness of mere repetition was influenced by neither species nor setting. These two studies demonstrated that orang-utans exposed to more sociable and terrestrial conditions evince remarkable behavioural plasticity, in that they produce additional innate or innovated signals, as well as more elaborate communicative repair strategies. Forth, we showed that orang-utan mothers not only differed in the composition of their infantdirected gestural repertoires and communicative tactics, but also in how they adjusted their communicative behaviour across social contexts, irrespective of research setting and species as well as other confounding variables. In sum, my findings underscored the importance of distinguishing between production and perception in studies of communication, and demonstrated that social tolerance, as a foundation for extended social interactions, plays a central role in the emergence of complex exchanges in great apes. Finally, my studies made a strong case for investigating within-individual variation: partitioning behavioural variation into its within-individual variation relative to between-individual and environmental sources of variation will allow us to estimate the extent plastic responses to the immediate environment in great ape communication.
Publications
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(2021). Multicomponent and multisensory communicative acts in orang-utans may serve different functions. Communications Biology 4: 917
Fröhlich, M., Bartolotta, N., Fryns, C., Wagner, C., Momon, L, Jaffrezic, M, Mitra Setia, T., van Noordwijk, M.A., & van Schaik, C.P.
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(2018). The function of primate multimodal communication. Animal Cognition 21: 619–629
Fröhlich, M. & van Schaik, C.P.
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(2019). Multimodal communication and language origins: integrating gestures and vocalizations. Biological Reviews 94: 1809–1829
Fröhlich, M., Sievers, C., Townsend, S.W., Gruber, T., & van Schaik, C.P.
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(2019). The loud scratch: a newly identified gesture of Sumatran orang-utan mothers in the wild. Biology Letters 15: 20190209
Fröhlich, M., Lee, K., Mitra Setia, T., Schuppli, C., & van Schaik, C.P.
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(2020). Must all signals be evolved? A proposal for a new classification of communicative acts. WIREs Cognitive Science 11: e1527
Fröhlich, M. & van Schaik, C.P.
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(2021). Orangutans have larger gestural repertoires in captivity than in the wild – a case of weak innovation? iScience 24: 103304
Fröhlich, M., Bartolotta, N., Fryns, C., Wagner, C., Momon, L, Jaffrezic, M, Mitra Setia, T., Schuppli, C., van Noordwijk, M. A., & van Schaik, C.P.
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(2022) Individual variation and plasticity in the infant-directed communication of orang-utans. Proceedings of the Royal Society B 289: 20220200
Fröhlich, M., van Schaik, C.P., van Noordwijk, M.A., & Knief, U.
