The mating system of callitrichids has been reported to be monogamous, polygynous and polyandrous. In Callithrix jacchus, groups with 2 breeding females and groups with 1 breeding female have been reported. Our purpose was to evaluate the occurrence of occasional reproduction by subordinate females in free-ranging C. jacchus groups characterized as monogamous. Four groups were monitored at a field site of IBAMA-Brazil for between 20 and 72 months. We recorded the birth of 7 sets of twins to subordinate females that had never reproduced before. Sexual activities were recorded opportunistically: dominant females copulated with only the resident male, while subordinates copulated with extra-group males. We suggest these were essentially monogamous groups that occasionally had 2 reproductive females. Between-group copulations seem to be an alternative strategy used by the subordinates. Despite the costs, there would probably be benefits as a result of scanning for vacancies for reproductive positionsin neighbouring groups and the establishment of ties with extra-group males that might become a reproductive partner in the future.

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This chapter presents data on the relations between reproduction and aging in both captive and free-ranging marmosets and tamarins. The relationship is examined from two perspectives. First, the relation of age to physiological impairments in reproductive function is explored. Callitrichid females, in common with many other nonhuman primates, begin to display anovulation associated with follicular depletion at a point relatively close to the maximum life span. Unlike Old World primates, however, they continued to display significant steroidogenic activity in the ovary. There are age effects on some reproductive output variables, such as litter size and inter-birth interval, though the effects are often small. Like other mammals, male marmosets display a change in levels of androgens with age, although the magnitude of the decrease is not large and they actually mount an elevated response to GnRH challenge as they age. We also examined whether age affects either the establishment or maintenance of a breeding position, the factors most important in determining lifetime reproductive success. Infant mortality did increase with increasing parturitions, suggesting that there may have been aging or parturition effects on lactation. Generally, marmoset females were well past the age of sexual maturity at the beginning of the tenure and approaching 8-9 years at the end of it. Reproductive decline did not appear to be a gradual process, but a rather abrupt one, often causing the dismantling of the group. There are potentially interesting relations among maternal age, mass and declining reproductive performance, given the known importance of mass as a determinant of reproductive success in female callitrichids.

Common marmosets are members of the family Callitrichidae, South American primates characterized by highly social group living and cooperative breeding. In this study we analysed 1112 base pairs (bp) of the mitochondrial control region in 59 Callithrix jacchus individuals, sampled mainly from two geographically distinct field sites in N.E. Brazil. Analysis of molecular variation revealed a highly significant genetic structuring of haplotypes between social groups and between populations. Examination of matrilineal genetic structure within social groups revealed that seven of nine recorded breeding pairs were from different maternal lineages, indicating assortative mating and outbreeding. In addition to the breeders, at least six of 10 groups contained adult individuals from different matrilines, with five haplotypes present in one social group of nine animals. Groups of mixed lineages raise questions about potential reproductive conflicts of interest, and the extent of kin-selected altruism in the evolution and maintenance of cooperative breeding in this species.