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Breaking conversational rules matters to captive gorillas: A playback experiment

Abstract : Across human cultures, conversations are regulated by temporal and social rules. The universality of conversational rules suggests possible biological bases and encourages comparisons with the communicative interactions of nonhuman animals. Unexpectedly, few studies have focused on other great apes despite evidence of proto-conversational rules in monkeys, thus preventing researchers from drawing conclusions on potential evolutionary origins of this behaviour. A previous study showed however that western lowland gorillas engage in soft call interactions that seem temporally-and socially-ruled. Indeed, interactions occurred mainly between individuals close in age who followed a preset response delay, thus preventing call overlap. Here, we experimentally investigated the presence of these rules in a captive gorilla group, using a violation-of-expectation paradigm. Head orientation responses suggest that the respect of response delay matters to subjects, but the importance of the interlocutors' age proximity appeared less clear. The intensity of the response varied with subjects' age in a context-dependent way, supporting a possible role of learning. Our findings support the growing number of studies highlighting the importance of vocal turn-taking in animals and a possible sociogenesis of this ability. The capacity to "converse" might have been a key step in the co-evolution of communication and complex sociality. Despite the diversity of human cultures, some basic conversational rules are respected in all societies and thus appear universal 1. These features are gathered in a so-called "contract of communication" that conversing interlocutors informally agree on 2. The contract takes into account "context relevance" (i.e. the evaluation of the context as pertinent or not to initiate a conversation), "reciprocity" (i.e. the evaluation of both partners as valid interlocutors), and "contract-based temporal rules" (i.e. the respect of a reciprocal exchange of alternating, short, and flexible turns between two or more interlocutors 3 , and speech overlap avoidance). Several authors have questioned the possible biological bases of these conversational rules and have, for several decades, conducted cross-species comparisons to understand their origin and role across animal communication systems 4-7. To facilitate cross species investigations, authors typically distinguish "conversation-like vocal exchanges" from other vocal patterns like isolated calling (one call emitted and no other calls can be heard around, e.g. in red-capped mangabeys Cercocebus torquatus 8), repeated calling (the same caller calls several times in a row, e.g. in Japanese macaques Macaca fuscata 9), disorganized phonoresponses (one individual produces a call, typically an alarm call that triggers calls in an apparent chaotic way from the other group members, e.g. in blue monkeys Cercopithecus mitis stuhlmanni 10), chorusing (two or more individuals overlap their emission of a given call type, e.g. in chimpanzees Pan troglodytes 11) and duetting (two individuals synchronise long series of calls or songs with stereotyped temporal association: e.g. in birds 12-14 , and gibbons Hylobates syndactylus 15). "Conversation-like vocal exchanges" are distinguished from the other types of vocal interactions because of their following specific features. First, they involve a diversity of recurrent vocal partners (which differs from duets). Second, interlocutors are typically familiar individuals belonging to a given social group and can be of any age or either sex (which differs from synchronized signalling 16-20). Third, "conversation-like vocal exchanges" are not restricted to a specific context, which differs from-usually long-distance-collective communication associated with mate attraction, territory protection and environmental disturbance 21 , and time of the day or year (excluding morning choruses and reproductive signals 22,23). Most commonly, these "conversation-like vocal exchanges" are observed
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Loïc Pougnault, Florence Levréro, Baptiste Mulot, Alban Lemasson. Breaking conversational rules matters to captive gorillas: A playback experiment. Scientific Reports, Nature Publishing Group, 2020, 10 (1), pp.6947. ⟨10.1038/s41598-020-63923-7⟩. ⟨hal-02553553⟩



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