Social drivers of vocal flexibility in female baboons

Many social animals produce signals during close encounters but, across species, there is considerable structural and functional variation, presumably driven by social and ecological factors. In this study, we explored how wild female olive baboons in Kibale National Park, Uganda, used grunts during encounters. We found that females directed their grunts selectively towards females with dependent offspring, while taking into account rank differences and, to a lesser extent, friendship. Specifically, females were more likely to grunt towards higher-ranking and weakly-bonded mothers than other females. Unlike Guinea baboons, chacma baboons and olive baboons from central Kenya, however, Ugandan olive baboons appeared not to use grunts to reassure subordinates but to appease the riskiest partner: a higher-ranking female with a young dependent offspring. Our data suggest that baboons have evolved the ability to use part of their vocal repertoire in population-specific, socially flexible ways. We tentatively propose that these population differences relate to variations in aggression rates, social structure and/or ecological variables.