Why do gorillas make sequences of gestures?
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Great ape gestures have attracted considerable research interest in recent years, prompted by their flexible and intentional pattern of use; but almost all studies have focused on single gestures. Here, we report the first quantitative analysis of sequential gesture use in western gorillas (Gorilla gorilla gorilla), using data from three captive groups and one African study site. We found no evidence that gesture sequences were given for reasons of increased communicative efficiency over single gestures. Longer sequences of repeated gestures did not increase the likelihood of response, and using a sequence was seldom in reaction to communicative failure. Sequential combination of two gestures with similar meanings did not generally increase effectiveness, and sometimes reduced it. Gesture sequences were closely associated with play contexts. Markov transition analysis showed two networks of frequently co-occurring gestures, both consisting of gestures used to regulate play. One network comprised only tactile gestures, the other a mix of silent, audible and tactile gestures; apparently, these clusters resulted from gesture use in play with proximal or distal contact, respectively. No evidence was found for syntactic effects of sequential combination: meanings changed little or not at all. Semantically, many gestures overlapped massively with others in their core information (i.e. message), and gesture messages spanned relatively few functions. We suggest that the underlying semantics of gorilla gestures is highly simplified compared to that of human words. Gesture sequences allow continual adjustment of the tempo and nature of social interactions, rather than generally conveying semantically referential information or syntactic structures.
KeywordsGreat ape Gestural communication Syntax Semantics Interaction regulation
This work was conducted as part of the European Commission Sixth Framework Programme “Origins of Referential Communication”, contract 12787. We thank WCS Congo for permission to work at Mbeli Bai, Congo, and in particular Emma Stokes and Mark Gately for essential logistical support, and Joel Glick for help in the field. Particular thanks are due to Thomas Breuer for allowing us to use video material on gorilla behaviour collected at Mbeli Bai. We are grateful to Basel and Zürich Zoos, Switzerland, and to La Vallée des Singes, France, for allowing us to study their excellent gorilla groups, and especially to Jakob Huber at Basel, Robert Zingg at Zürich, and Jan Vermeer at La Vallée, who gave us help throughout. Peter Slater gave us valuable help towards appropriate analyses and interpretation. We thank Erica Cartmill, Barbara King, Katja Liebal, Joanne Tanner and three anonymous referees for many helpful comments on an earlier draft of this paper.
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