Sanje mangabeys mate year-round. A female copulates with multiple adult males from her group. Lower-ranking males have a better chance of “getting lucky”—that is, experiencing greater mating success—when a group’s number of receptive females increases. Known as female synchrony—the greater number of ovulating females—the more challenging it is for a dominant male to succeed in monopolizing a harem all to himself. He is simply unable to simultaneously tend to each fertile female, unavoidably inviting breeding competition from other males in his group. (Conversely, when the number of receptive females is small, dominant males have the breeding advantage.)

Mating behavior and male mating “skew” was the subject of a 22-month study (published in 2017) that examined the impact of two outside Sanje mangabey males who infiltrated and took over an established group. Prior to the takeover, the group’s dominant male was “copulator-in-chief” (responsible for 75 percent of copulations with the females of his harem). But the arrival of the two outsider males, who achieved dominance, caused a temporary instability—a skew—in  in the group’s hierarchy when they became chief copulators of the harem.

When a female is in estrus (the period when she is capable of conceiving), her genital area swells, indicating to her male suitor that she is physically ready to copulate. She presents her rear end to the male and may wave her head. Enticed, the male sniffs or touches the female’s genitals before mounting her. He mounts her by holding onto her ankles with his feet while grabbing her hips with his hands. (Researchers have dubbed this technique the “double-foot clasp.”) The female may arch her head back and make eye contact with her male partner, or she may reach back to grasp his leg, shoulder, face, or hair.

Soon after she copulates with one male, a female emits a pre-copulatory call to attract her next partner. She may mate with as many as seven males in one day! Whether her behavior is considered promiscuous or is a means to ensure conception—males are only single-mount ejaculators—is a topic open to scientific speculation (or gossip!). A female’s “promiscuousness” might also be savvy feminine strategy. Having multiple male partners makes paternity questionable, which, in turn, may reduce the occurrence of infanticide and may garner help with parental care from a fecund female’s male suitors.

Gestation period (pregnancy) in the species is between 5-1/2 and 6 months.

Mangabeys give birth to a single offspring, who is born with soft fur and with eyes open.

Like all mangabeys, infant Sanje mangabeys cling to their mother’s belly as she travels. Between 3 and 5 weeks of age, infants begin to grasp and mouth objects. At 12 weeks they begin engaging in social play and can walk, run, and jump. Young mangabeys are considered weaned between 7 to 10 months of age, but they remain near their mother until she gives birth to a new sibling.

Male Sanje mangabeys participate in co-parenting by carrying and protecting a group’s infants (this shared infant care is known as “alloparenting”). But they might have an underlying motive to their attentiveness. Apparently, males who more often help with infant care are granted more opportunities by the females to copulate.