Close encounters of the friendly kind: pacific between-group interactions in primates.

While intergroup conflict features prominently in the behavioural ecology literature, its antonym, intergroup peace, has been a rather neglected phenomenon until recently. Neighbourly relations and affiliative interactions are far from uncommon. We provide a comprehensive synthesis of the empirical evidence of peaceful between-group interactions in primates which take various forms that vary along two continua: intensity and duration. These include intergroup tolerance, intergroup affiliation, visits, mingling, intergroup coalitions and mergers between groups. We propose that an analysis of intergroup peace would benefit from distinguishing between facilitating factors (facilitators), active drivers and stabilizing forces (stabilizers). A prime example of a facilitator is resource heterogeneity in the landscape which can make resource defence logistically impractical and make initially independent groups more interdependent. While this often suffices for tolerant associations to emerge, for full-blown intergroup peace with sporadic or frequent intergroup affiliation and/or cooperation to arise, motivations or adaptive benefits (drivers) need to be present. Some of the adaptive benefits that can trigger a switch from tolerance to peace between groups include dispersal facilitation, information gathering, predation protection, extra-group mating, communal defence, and reciprocal resource access. Lastly, a further mediator of xenophobia and intergroup agonism is the degree of relatedness and familiarity between interacting groups (stabilizers). Finally, we propose several ways to move this research forward. We hope that this review will stimulate empirical and theoretical studies and encourage field researchers to pay more attention to hitherto rather neglected forms of between-group contact. An understanding of the functional correlates of peaceful between-group relationships in primates also holds promise for making inferences about the human social system where intergroup peace has enabled cooperation and cultural diffusion on an unprecedented scale.