Predation risk places a pressure on animals to adopt mechanisms by which they reduce their individual risk of being preyed on. However, a consensus on methods of determining predation risk has yet to be reached. One of the most widespread ways in which animals respond to predation risk is by living in groups. Minimum permissible group size is the smallest group size that animals are able to live in, given the habitat-specific predation risk they face. We explore ways in which predation risk can be measured and analyse its effect on minimum observed group size in baboons. Using data on predator density, habitat composition and baboon body size, we investigate the impact of the components of predation risk on baboon group size, and derive an equation that best predicts minimum group size. Minimum group size in baboons is related to predator density and female body mass. Both of these elements can, in turn, be estimated from environmental variables. These findings present support for the argument that group living in primates is a response to predation risk and offer potentially new ways of investigating carnivore and primate ecology.

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