Vermeer, J. 1997. The formation of a captive squirrel monkey group. International Zoo News 44, 146-149.

It is important that all females of a new [heterosexual] group are related to each other, that is, that they come from the same natal group. The introduction of unfamiliar females to a small group with several females can result in much aggression with severe injury. Sometimes such unfamiliar females are tolerated by other group members, but they will hardly ever become fully a part of the group, and there can be stress for long periods. The minimum number of breeding females in a group should be five to seven. A maximum of two adult males should be added to these females. Additional adult males will not be accepted by the others in the group. Two adult males may tolerate each other, but preferably these two should be related animals, e.g., brothers. Castration does not seem to be successful as means of controlling aggression. Groups of up to ten males can be formed without many problems. Preferably, these males should come from the same natal group, where they were used to living together in one of the sub-groups.