How life in a tolerant society affects the usage of grunts: evidence from female and male Guinea baboons

Signals are used to regulate interactions between individuals. To disentangle how motivational disposition, the processing of social information, and the costs and benefits of putative outcomes of interactions affect signalling behaviour, we investigated the usage and function of grunts during approaches in wild Guinea baboons, Papio papio. Guinea baboons live in a tolerant multilevel society with female-biased dispersal, which allowed us to compare their grunt usage to that of other more despotic baboon species. We analysed approaches by female and male Guinea baboons living in the Niokolo Koba National Park, Senegal. When approaching baboons grunted, they were more likely to interact in an affiliative fashion and less likely to displace the partner. In females, the probability of grunting was higher when the relationship strength was low, but only when an infant was present. In males, relationship strength had no impact on the likelihood of grunting during approaches. Rank did not explain variation in grunt probability in females and could not be discerned in males, but males were also more likely to grunt when an infant was near a female partner. We suggest that grunt usage in baboons can be best conceived as a combination of a motivational and a strategic component. The motivational component expresses the increased disposition to interact in an affiliative fashion, while the strategic component refers to the modulation of grunt usage with regard to relationship quality and context. The motivational component appears to be shared between baboon species, while variation in despotism and social organization places different premiums on the benefits of signalling, resulting in variation in grunting patterns between species.