Keeping in Contact: Flexibility in Calls of Olive Baboons

Vocal production learning, and in particular the ability to imitate other people’s vocalisations, is a key foundation of human speech. From an evolutionary perspective, it is puzzling that our closest living relatives, monkeys and apes, show so little flexibility in terms of altering the structure of their vocalisations. Instead, non-human primates typically have a restricted vocal repertoire consisting of different call types with varying degrees of variability within and between types. This raises two questions: first, which factors affect the overall morphology of vocal repertoires, and second, within the rather fixed system of a given vocal repertoire, which aspects of vocal production may reveal a certain degree of plasticity. Baboons lend themselves for an investigation of these questions because they show a high degree of flexibility with respect to social structure and habitats they range in. Moreover, their phylogenetic relatedness is well understood. In this study, we investigated whether olive baboons from two troops ranging in Gashaka Gumti National Park / Nigeria, adjust their calls in relation to the habitat. We compare the results to findings from other sites (olive baboons from Uganda, chacma baboons from Botswana), to explore variation within and between populations. We focus on contact calls used over short distances (grunts) and long distances (clear barks) and tested whether usage (grunt and bark rates) and call structure differed in relation to the habitat. We expected a larger degree of flexibility in call usage compared to variation in call structure and predicted that subjects would call more frequently when the visibility is poor. If individuals are in fact able to modify the structure of their calls, theory predicts that they should optimise the propagation distance by using longer calls in forest compared to woodland-savannah, with a lower ­frequency and energy concentrated in lower frequencies. Indeed, the baboons uttered significantly longer grunts in forest than in open woodland, suggesting some degree of intra-individual short-term flexibility. Contrary to our expectations, grunt usage did not vary with the habitat type, perhaps because a large proportion were used in social contexts, during infant handling and friendly approaches. Since these calls are given at close range, visibility was not affected. Our observation also showed little variation in grunt rate between the troops. However, compared to some other populations and taxa, Nigerian baboons grunted rarely, so that the lack of variation in relation to habitat may simply reflect a floor effect. Clear barks were given mostly as single calls by immature individuals and adult females when separated from the group or particular group members, while resting alone and travelling. Bark rate varied between troops, but not in relation to habitat type. Due to small sample size, bark structure was not analysed. Overall, our findings highlight similarities between baboon taxa in call contexts, as well as variation in responses to changing environmental conditions. Probably, other factors than the environment – such as interaction rates, for instance – may affect call usage. Future studies will need to integrate data from multiple baboon taxa to establish a better picture of the interplay between different factors that govern variation in call usage and structure.