We investigated the mechanism of alliance formation among adult male savanna baboons by comparing the characteristics-of males that formed coalitions frequently with males that never or seldom took part in coalitions. We observed three groups: two of Papio cynocephalus cynocephalus in Amboseli National park Kenya, and one of P. c. anubis in the vicinity of Gilgil, Kenya. We considered four hypotheses: (1) Males must be familiar with each other, (2) males must have an affinitive bond, (3) males must have move than average experience, and (4) the combined fighting ability of the coalition partners relative to the fighting ability of their opponent determines the likelihood that a coalition is formed. We conclude that relative fighting ability forms the key factor in coalition formation. High-ranking males do not form coalitions often, since they hardly need them. Low-ranking males rarely form coalitions, since they cannot form effective coalitions among themselves. Affinity (''friendship'') may play a role as an additional factor. The relation of coalition formation with age and period of residence, which was found in several studies, can be explained largely by the correlation between these parameters and fighting ability.
