Dominance competition among siblings in Japanese macaques: constraints on nepotism

Abstract

Abstract. Matrilineal rank orders, common in cercopithecine monkeys, are often called nepotistic because female kin occupy adjacent ranks and support each other preferentially. But alliances among non-kin also play a central role in these dominance systems, enabling females to maintain their rank above lower-ranking individuals. The aim of this study was to investigate the extent to which nepotism (among siblings) might conflict with a female's need to form alliances with non-kin. The study was conducted at the University of Montreal Laboratory of Behavioural Primatology housing a group of 35 Japanese macaques, Macaca fuscata. In this species females outrank their older sisters after a period of aggressive challenge. Younger siblings that were individually unable to maintain their rank above an older sister were found to be able to do so in the presence of dominant non-kin, indicating that the latter are effective allies for younger siblings. Thus, in conflicts between non-kin allies and their older sisters, younger siblings may face a conflict of interest between siding with the non-kin ally to increase their competitiveness against their sister, or help their sister. One thousand and seventy-seven aggressive interventions involving 22 sibling dyads over a 56-month period were analysed. In accordance with the experimental results, dominant non-kin supported preferentially the younger siblings. The latter appeared divided between siding with their older sister and the non-kin ally. However, in conflicts between non-kin individuals and kin that they did not target for dominance, younger siblings acted nepotistically more often. Thus, nepotism appeared constrained by a female's need for non-kin allies. These data are best interpreted in terms of individual advantages rather than in terms of kin selection or reciprocal altruism.