Influences of ecological factors on vocal communication in olive baboons (Papio hamadryas anubis)

Abstract

In non-human primates, flexibility is largely documented in usage of vocal signals, while it appears more limited in vocal production. The present thesis adopts two perspectives to investigate the degree of flexibility in usage and structure of baboon contact calls -namely grunts, clear barks and loud calls-, which is expected to be highest in usage. First, variability was examined within and between baboon taxa from an evolutionary perspective. Audio-recordings and behavioural data collected in two troops of olive baboons (Papio hamadryas anubis) in Nigeria were compared with data available from other baboon populations and taxa. Second, the degree of intra-individual flexibility was evaluated with a focus on ecological factors. A review of studies in anurans, birds and mammals tested hypotheses concerning environment-related variations in vocal behaviour, and baboon habitat was structurally and acoustically characterised. Then, intra-individual flexibility in response to environmental variations was examined in data on vocal behaviour of the Nigerian olive baboons combined with data from another olive baboon troop in Uganda. Between olive baboon populations and between baboon taxa, emission rates of grunts, clear barks and loud calls varied importantly, while their contexts of utterance and acoustic structures underwent only minor differences. The review of studies in anurans, birds and mammals revealed that environment-related adjustments in vocal behaviour at a species or population level were generally in line with predictions based on sound propagation features but not as widespread as expected. Since it also underlined the importance of a precise environmental description, closed and open habitats were characterised in the home ranges of the studied olive baboons. Each individual adapted grunt duration to visibility conditions in Nigeria and in Uganda, while grunt rate was adjusted in Uganda only. Contexts of calling and proximity to group members nevertheless constrained this intra-individual flexibility. Regarding vocalisations used over longer distances, baboons tended to avoid high ambient noise level to utter most loud calls. In contrast, emission rate of clear barks seemed to be regulated mainly by other factors than the environment, like predation risk and caller’s arousal. Overall, the evolutionary approach confirms a larger flexibility in usage than in structure of vocalisations between baboon populations and taxa. Surprisingly, intra-individual flexibility in response to environmental variations occurs systematically in grunt production but not in usage of short- and long-distance contact calls. The fine control on vocal production in speech might have evolved from this precursory intra-individual plasticity in a common ancestor with humans.