Habitat-dependent intergroup hostility in Diana monkeys, Cercopithecus diana

Territorial threat is costly and variable across contexts and behavioural flexibility is favoured to maximize any cost/benefit ratio. This is well illustrated in how animals react to familiar or unfamiliar outgroup members. In some situations, neighbours are better tolerated than strangers, resulting in a ‘dear-enemy effect’; in other situations, the pattern is reversed, resulting in a ‘nasty-neighbour effect’. Typically, the effects are species-specific traits, although both can also occur within the same species. Here, we investigated wild Diana monkeys of Taï Forest (Ivory Coast) in their reactions to outgroup individuals using playbacks of both familiar and unfamiliar male alarm calls to eagles. We found that groups living in primary forest (high group density, high food availability and low predation pressure) followed a ‘nasty neighbour’ strategy whereas groups living in secondary forest (low group density, low resources and high predation risk) followed a ‘dear enemy’ strategy, suggesting that group density, predation pressure and food availability can impact how hostile behaviour is displayed in nonhuman primates. Our results confirm a high behavioural flexibility in primate relationships between conspecifics of different identities depending on ecological traits of the habitat.