Thomson's gazelles, Gazella thomsoni, and Grant's gazelles, G. granti, in the Serengeti National Park, Tanzania, frequently associate in mixed-species groups. Males are more likely to associate than females. Here the antipredator benefits of such associations are investigated. Compared with remaining as a smaller group of conspecifics, joining Grant's gazelles to form larger mixed-species groupss had several advantages for Thomson's gazelles. First, they were less vulnerable to cheetahs, Acinonyx jubatus, one of their main predators, as a result of improved predator detection, the lower success rate of cheetah hunts and the tendency for cheetahs to avoid hunting larger groups. Second, the Thomson's gazelles in mixed-species groups shared vigilance to some extent with the Grant's gazelles, leaving more time available for feeding. However, compared with joining conspecifics, joining Grant's gazelles to form mixed-species groups had few additional advantages and such associations were likely to be beneficial only when the number of conspecifics in the resulting mixed-species groups was relatively high. Associating with Grant's gazelles did not, for example, enable individual Thomson's gazelles to spend less time vigilant than associating with the same number of conspecifics. Grant's gazelles benefited from the association because the cheetahs' preference for the smaller Thomson's gazelles reduced the rate at which the Grant's gazelles were attacked when in mixed-species groups. © 1990 The Association for the Study of Animal Behaviour.
