Behaviour of adult and immature male baboons during inter-group encounters

Abstract

SOCIAL interaction between groups of Old World monkeys varies from peaceful mingling to marked aggression^1–3. One constant feature, however, concerns the differing behaviour of immature and adult males; and these differences have been related to differences in male reproductive success at different stages of the life cycle. Immature and young adult males in multi-male groups of baboons and macaques frequently approach and interact with individuals outside their natal group, sometimes transferring from one group to another^4–7. Two suggested causes of such group transfer are inability to gain access to oestrous females in the natal group and attraction to oestrous females in other groups⁸. In contrast, adult males—both in species with one-male social units and in species with multi-male groups—are frequently aggressive during intergroup encounters, often chasing or ‘herding’ members of their own group away from other groups^7,9,10. Herding usually involves high-ranking adult males who are also the individuals who copulate most often^10,11. Males are most likely to herd adult females, but rarely herd females who are lactating^10–12. (Although occasional herding in gelada baboon one-male units has been reported¹³). Herding has been suggested both to mobilise individuals before group movement⁷ and to maintain a male's access to oestrous females over time¹⁰. To investigate the function of herding we observed the intergroup encounters of one multi-male group of baboons for 15 months. Our observations suggest not only that herding is correlated with differences in male reproductive success but also that it is related to changes in the female reproductive cycle and to long-term bonds between particular males and females.