Social pressure drives "conversational rules" in great apes.

In the last decade, two hypotheses, one on the evolution of animal vocal communication in general and the other on the origins of human language, have gained ground. The first hypothesis argues that the complexity of communication co-evolved with the complexity of sociality. Species forming larger groups with complex social networks have more elaborate vocal repertoires. The second hypothesis posits that the core of communication is represented not only by what can be expressed by an isolated caller, but also by the way that vocal interactions are structured, language being above all a social act. Primitive forms of conversational rules based on a vocal turn-taking principle are thought to exist in primates. To support and bring together these hypotheses, more comparative studies of socially diverse species at different levels of the primate phylogeny are needed. However, the majority of available studies focus on monkeys, primates that are distant from the human lineage. Great apes represent excellent candidates for such comparative studies because of their phylogenetic proximity to humans and their varied social lives. We propose that studying vocal turn-taking in apes could address several major gaps regarding the social relevance of vocal turn-taking and the evolutionary trajectory of this behaviour among anthropoids. Indeed, how the social structure of a species may influence the vocal interaction patterns observed among group members remains an open question. We gathered data from the literature as well as original unpublished data (where absent in the literature) on four great ape species: chimpanzees Pan troglodytes, bonobos Pan paniscus, western lowland gorillas Gorilla gorilla gorilla and Bornean orang-utans Pongo pygmaeus. We found no clear-cut relationship between classical social complexity metrics (e.g. number of group members, interaction rates) and vocal complexity parameters (e.g. repertoire size, call rates). Nevertheless, the nature of the society (i.e. group composition, diversity and valence of social bonds) and the type of vocal interaction patterns (isolated calling, call overlap, turn-taking-based vocal exchanges) do appear to be related. Isolated calling is the main vocal pattern found in the species with the smallest social networks (orang-utan), while the other species show vocal interactions that are structured according to temporal rules. A high proportion of overlapping vocalisations is found in the most competitive species (chimpanzee), while vocal turn-taking predominates in more tolerant bonobos and gorillas. Also, preferentially interacting individuals and call types used to interact are not randomly distributed. Vocal overlap ('chorusing') and vocal exchange ('conversing') appear as possible social strategies used to advertise/strengthen social bonds. Our analyses highlight that: (i) vocal turn-taking is also observed in non-human great apes, revealing universal rules for conversing that may be deeply rooted in the primate lineage; (ii) vocal interaction patterns match the species' social lifestyle; (iii) although limited to four species here, adopting a targeted comparative approach could help to identify the multiple and subtle factors underlying social and vocal complexity. We believe that vocal interaction patterns form the basis of a promising field of investigation that may ultimately improve our understanding of the socially driven evolution of communication.

Experimental Tests for Measuring Individual Attentional Characteristics in Songbirds.

Attention is defined as the ability to process selectively one aspect of the environment over others and is at the core of all cognitive processes such as learning, memorization, and categorization. Thus, evaluating and comparing attentional characteristics between individuals and according to situations is an important aspect of cognitive studies. Recent studies showed the interest of analyzing spontaneous attention in standardized situations, but data are still scarce, especially for songbirds. The present study adapted three tests of attention (towards visual non-social, visual social, and auditory stimuli) as tools for future comparative research in the European starling (Sturnus vulgaris), a species that is well known to present individual variations in social learning or engagement. Our results reveal that attentional characteristics (glances versus gazes) vary according to the stimulus broadcasted: more gazes towards unusual visual stimuli and species-specific auditory stimuli and more glances towards species-specific visual stimuli and hetero-specific auditory stimuli. This study revealing individual variations shows that these tests constitute a very useful and easy-to-use tool for evaluating spontaneous individual attentional characteristics and their modulation by a variety of factors. Our results also indicate that attentional skills are not a uniform concept and depend upon the modality and the stimulus type.

When perceptual laterality vanishes with curiosity: A study in dolphins and starlings.

Sensory laterality is influenced by the individual's attentional state. There are variations in the way different individuals of a same species attend to stimuli. When confronted to novelty, some individuals are more explorative than others. Curiosity is composed of sensation and knowledge seeking in humans. In the present study, we hypothesized that more curious animals, i.e., showing more sensory exploration would be less lateralized than quietly attentive individuals, performing instead more gazing behaviours. In order to test this hypothesis and its possible generality, we performed two studies using two animal models (dolphins and starlings) and two modalities (visual and auditory) of presentation of species-specific and non-species-specific stimuli. Both dolphins and starlings presented more gazes for the species-specific stimuli and more exploratory components for the non-species-specific stimuli. Moreover, in both cases, the non-species-specific stimuli involved more lateralized responses whereas there was no or less clear laterality for the species-specific stimuli. The more exploratory dolphins and starlings also showed a decreased laterality: the more "curious" individuals showed no laterality. Further studies are needed on characterization of curiosity in relation to attention structure. The present study suggests that individual variations in sensory laterality may help disentangle the subtle differences between curiosity, attention and boldness.

Breaking conversational rules matters to captive gorillas: A playback experiment.

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.