Red colobus and Diana monkeys provide mutual protection against predators.

We tested the hypothesis that red colobus monkeys, Procolobus badius and Diana monkeys, Cercopithecus diana in Taï National Park, Ivory Coast, associate to increase safety from predation. We recorded stratum use, exposure and vigilance of each species in the presence and in the absence of the partner species. In the presence of Diana monkeys, red colobus used lower strata more often, were more exposed to the forest floor and looked down less often while foraging. This suggests that red colobus reduce predation pressure from ground predators by associating with Diana. There are several indications that both species are less vulnerable to birds of prey when associated: red colobus looked sideways less often and were more exposed to the front, Diana monkeys used greater heights and were more exposed to the front, to the rear and from above. We also asked whether specific sentinel qualities of Diana monkeys might explain why other sympatric monkey species additionally seek their presence. When groups of various species compositions were approached by an observer, or confronted with an eagle model, Diana monkeys raised the alarm in most cases. Since neither monkey species improves its foraging success when associated, this study shows that predation can both maintain and be the ultimate cause of interspecific associations.Copyright 1997 The Association for the Study of Animal Behaviour