A detailed comparison of Macaca fuscata and Papio anubis reveals that female aggression to males has an effect on male association with females in both species, but only in M. fuscata do females appear to be able to limit the number of males in their troop. It is suggested that this is the reasons why 'solitary' males are commonly reported in M. fuscata, but rare in P. anubis, and may be related ultimately to the different degrees of sexual dimorphism in the two species.