Why mob? Reassessing the costs and benefits of primate predator harassment.

While some primate species attempt to avoid predators by fleeing, hiding or producing alarm calls, others actually approach, harass and sometimes attack potential threats, a behavior known as 'mobbing'. Why individuals risk their safety to mob potential predators remains poorly understood. Here, I review reports of predator harassment by primates to (1) determine the distribution of this behavior across taxa, (2) assess what is known about the costs of mobbing, and (3) evaluate hypotheses about its function. Mobbing is taxonomically widespread and is used against a wide range of predator species. However, inconsistent use of the term 'mobbing' within the primate literature, the lack of systematic studies of primate mobbing, and the likelihood of systematic biases in the existing data pose significant obstacles to understanding this puzzling behavior. Although difficult to quantify, the costs associated with harassing predators appear nontrivial. Many benefits that have been proposed to explain mobbing in birds may also be important in primate systems. There are puzzling aspects of primate mobbing, however, that existing hypotheses cannot explain. Future research should consider the within-group signaling potential of this costly behavior, as well as the ability of behavioral syndromes to explain the distribution of mobbing in primates.