Natal male baboon rank rises and successful challenges to resident alpha males

Summary

After an interval (X = 6 months), high ranking male chacma baboons (Papio cynocephalus ursinus) lose their status to immigrants. Attainment of alpha rank by immigrants is a qualitatively different process from thestepwise increase in status noted in linear dominance hierarchies. The departing rank of natal emigrants was 5.4, while the first measured rank, shortly after transfer into a new troop, was 1.5. Abrupt rises to alpha rank involve direct challenges to the current alpha male. Fourteen of 19 prime immigrants attained alpha rank at first transfer. Rank rises may result if individuals forego contests, retaining lower hierarchical positions in their natal troop to avoid the costs of conflict prior to natal emigration. In this population some males do breed in their natal troops without differing from immigrants in their reprodocutive success. Five males that rose to high rank within their natal troops also made rapid rank rises to the top of the hierarchy. Withholding of aggressive efforts in natal troops to avoid inbreeding thus is not an adequate explanation of the rank rises we observe in this population. The high probability of successful challenges can be explained by assuming an asymmetry in costs of losing (resource value) to tenured alpha males or a greater fighting ability of challengers.