Tamarins and marmosets (subfamily: Callitrichinae) are members of an ecologically and behaviorally diverse radiation of small-bodied New World monkeys characterized by the production of twin offspring, intense female reproductive competition, reproductive suppression, cooperative infant care and evidence of what has been described as a highly flexible system of mating. In this review, I offer a phylogenetic approach to the study of callitrichine reproduction and mating systems. Beginning with the assumption that Callimico most closely resembles the ancestral tamarin and marmoset condition, I identify a set of primitive callitrichine behavioral and reproductive character states and reconstruct the sequence of derived behavioral and reproductive changes that define the basic social systems of Saguinus, Leontopithecus, Callithrix, and Cebuella. It is proposed that ancestral callitrichines lived in small multimale multifemale groups characterized by 1 or more reproductively active females, a single offspring at birth, rapid postnatal growth, delayed nonmaternal caregiving, and a polygynous mating pattern. The evolution of twinning in the common ancestor of all extant tamarins and marmosets increased the overall costs of infant care, resulting in enhanced female reproductive competition and a greater role for group members in providing for the young.
Specialized mechanisms of female reproductive competition appear to have evolved twice in callitrichine evolution. In the lineage leading to Leontopithecus, high levels of female intrasexual aggression and ovulatory synchrony play a primary role in the social control of fertility. In Saguinus, Callithrix, and Cebuella, female reproductive suppression has a stronger physiological basis, resulting in hormonally mediated infertility in subordinate adult females. In addition to having important implications for understanding differences in callitrichine breeding systems, these data suggest that Leontopithecus separated from the callitrichin line prior to the radiation of other tamarin and marmoset taxa. © 1994 Wiley-Liss, Inc.