Effect of auxiliary males on territory ownership in the oribi and the attributes of multimale groups

I observed free-ranging oribi, Ourebia ourebi, in Serengeti National Park, Tanzania, to determine whether group formation by males provides evidence f or cooperative territory defence, a behaviour that is rare among male verte brates. Socially dominant males that shared territories with subordinate au xiliary males were replaced by rivals less often than males that defended t erritories without auxiliary males. Auxiliary males marked territories with preorbital glands, dung and urine, and territories defended by male groups were marked more thoroughly than those defended by single males. Fifteen o f 24 (62.5%) auxiliary males whose histories were known were born on territ ories defended by males that probably were their fathers. But 37.5% of auxi liary males probably were unrelated to dominant males, because male groups also formed when territory owners accepted adult male immigrants as subordi nates, and when owners allowed young males to remain philopatric after evic ting a male likely to have been the young male's father. All males in group s probably had some mating access to females, but dominant males may have m inimized matings of auxiliary males by guarding fertile females. These resu lts suggest that auxiliary male oribi aided dominant males in territory def ence, and that dominants traded off the risks of losing matings to auxiliar ies, or being overthrown by them, in exchange for a reduction in their chan ce of being evicted by rival neighbours or immigrants. (C) 1999 The Associa tion for the Study of Animal Behaviour.