Monogamy is rare in cercopithecine monkeys; most are polygamous.
Single births are typical, although twins are born very occasionally, and two teats (mammae) are consistently present in the chest region. All species have a menstrual cycle lasting approximately a month and marked by externally visible menstrual bleeding. In many species, females have a conspicuous sex skin in the genital region that changes in coloration and size over the course of the ovarian cycle. Maximum swelling and the most prominent degree of coloration (commonly bright red) occur around the time of ovulation, approximately halfway between menstrual episodes. Conspicuous sex swellings are found in baboons, mandrills, some mangabeys, short-tailed macaques, swamp monkeys and talapoins, but they are less prominent or virtually absent in long-tailed macaques and forest-living guenons and other mangabeys. Geladas are highly unusual in that the sex skin is located on the chest, as a patch bordered by vesicles that varies in color and prominence over the ovarian cycle. This special development is undoubtedly linked to the fact that geladas spend much of the day shuffling along on their hindquarters while foraging. Placentation is of a highly invasive hemochorial type. The gestation period is long, varying between 162 days for the smallest species, the talapoin (Miopithecus talapoin), and 187 days for one of the largest, the chacma baboon (Papio ursinus).
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