Many cooperatively breeding species exhibit high reproductive skew. Delayed dispersal and cooperative breeding may have evolved as a consequence of the limits ecological constraints place on independent breeding. When simultaneous breeding by multiple females reduces the survival of the dominant's offspring, selection should favor dominants able to control subordinate reproduction. Monopolization of reproduction by dominant group members by means of suppression of subordinate reproduction has been documented in several taxa of cooperative breeders. In this dissertation I examine the proximate mechanisms and ultimate causes of reproductive skew in cooperatively breeding golden lion tamarins (Leontopithecus rosalia). In chapter one I combine data from phenological transects with hormonal evaluation of reproductive status to test whether caloric availability limits reproduction by female tamarins. Caloric availability was sufficient to support not only pregnancy polygyny in 83% of tamarin groups, but also the additional group members resulting from multiple litters. The super abundance of calories and the timing of births suggested that competition for allocare rather than for food resources may be the selective force limiting reproduction by subordinate females. In my second chapter I combine hormonal and demographic data to explain reproductive skew in terms of the costs and benefits to subordinate fitness under existing social circumstances. Subordinate females older than 18 months of age ovulated while residing within their natal group, but conceptions were delayed one to two years following reproductive maturity. The likelihood of successful reproduction by a subordinate female doubled with each year of age of the dominant female. Conceptions under incestuous mating conditions were rare (7 of 37 pregnancies). My results provide support for the hypothesis that subordinate adult females under three years old exercise reproductive self-restraint. I speculate that the threat of being evicted from the group and inbreeding avoidance are sufficient to delay attempts at reproduction by young subordinates without the need for costly fighting with the dominant female. In my third chapter I test whether reproduction by subordinate adult female tamarins is limited by dominant females who have incomplete control (incomplete control model, ICM) or complete control (optimal skew model, OSM) over subordinate reproduction. I combine hormonal data with group demography and caloric availability to determine variables useful in predicting a successful pregnancy to a subordinate female. Whereas subordinate females younger than 2.5 years of age ovulated but did not conceive, all females older than 3.9 years of age became pregnant. Reproduction in subordinate adult females was not limited by hormonal suppression of ovulation or conception, but by the failure of 7 of 11 pregnancies to produce live offspring. The likelihood of reproductive success increased 1.7 times with each additional group member. My results suggest that when caloric availability is sufficient to support reproduction by two breeding females and the group members necessary to provide allocare for two litters, subordinate females do not abide by a social contract that would limit their reproduction (OSM). Instead, older subordinates compete with dominant females for reproduction and succeed in producing live young if the dominant female is at least 10 years old, if subordinates conceive while the dominant is heavily pregnant, and if they reside within larger groups (ICM).