Like other primate cooperatively breeding species, pygmy marmoset groups generally have one dominant breeding female while the other adults will remain in the group without breeding. This is to insure the best genes are preserved, according to Darwin’s theory of natural selection. The sub-adults will stay within the family unit to help rear and raise the children. Sub-adults are deprived of the ability to reproduce in the groups, there is only one dominant female. Female Pygmy Marmosets will reach sexual maturity between 15 and 17 months of age, but if she is not the dominant female she will be suppressed of reproduction by interactions of the dominant female in the group. If the breeding female disappears from the group, her oldest daughter will become the next dominant female. In nature when an offspring of the sub-females gives birth, they are often victims of neglect or infanticide by other group members. Males will reach sexual maturity at around 16 months of age.
Adult males are interested in mating with the dominant female throughout the year, but the breeding female is not receptive to their advances during pregnancy or during the three to six weeks after parturition. The dominant male to establish his dominance, will aggressively intervene between any males attempting to mate with the dominant female. Court ship behaviors of the dominant male include approaching and following the female, strutting, tongue-flicking, sniffing and licking the females urine, scent marking using their glands, huddling, and grooming. If interested the female will respond to these behaviors by presenting their genitals to the male, raising their tails into an arch position, scent marking, huddling, and grooming.
Pygmy Marmosets do not have birth seasons but there are two birth peaks during the year, in the months of May and June and between November and January. Females will produce two litters each year and give birth to twins about 70% of the time in the wild. In captivity twins are born 76% of the time, triplets 8%, and singles 16%.