 Cooperative breeding entails one or more individuals, usually females, acting as "helpers" to one or a few dominant female breeders, usually helpers' kin.This sociosexual system is rare in primates, so far demonstrated among Neotropical callitricids, including marmosets and tamarins.Cooperative breeding requires "repression" of helpers' reproduction, by pheromones emitted by a breeder, by coercion, or by self-restraint.