1. Data collected on patterns of mating, dispersal, and reproduction in an island population of moustached tamarin monkeys (Saguinus mystax) indicate that these callitrichines; live in small multimale multifemale social groups. The adult composition of social groups was highly variable, ranging from 1-4 adult females and 1-3 adult males. None of the 13 completely censused groups contained a single adult male and a single adult female at the time of capture or during subsequent censuses.
2. All 6 groups for which data are available were characterized by either a polyandrous or a polygynous (polygyandrous) mating system.
3. In all groups with more than 1 female it is only the oldest female that produces offspring. Most of these females are known or estimated to be at least 8 years of age. Young (2-4 years of age) and middle (4-8 years of age) aged females rarely reproduce. The mating system of this tamarin species is best described in terms of age-dependent female reproductive sovereignty. Based on both morphological and behavioral information, there was no evidence that reproductive activity in male moustached tamarins vas similarly age dependent.
4. In this population migration is common, and adults and subadults of both sexes individually transfer into and out of groups. There is also evidence that males often migrate as pairs into the same social group. Twenty-three percent of the groups on the island contained a paired set of immigrant males. These males remain together in this new group for up to 8 years. We have no unambiguous cases of paired female migration. It is likely that male kinship/social bonds form the basis for much of the cooperative infant care exhibited by these tamarins.
5. We conclude that there is no compelling evidence to support the contention that the modal mating system of moustached and other tamarin species is monogamous, and offer the possibility that mating system flexibility and cooperative infant care in callitrichines evolved from a polygynous mating pattern.
