Female monkeys do experience periods, similar to human women. This biological process, characterized by the shedding of the uterine lining, is a shared trait among certain primate species, including humans and specific groups of monkeys. Understanding this phenomenon provides insights into reproductive biology across the primate order.
The Primate Menstrual Cycle
Menstruation in primates is a distinct phase within the reproductive cycle, occurring when pregnancy does not take place. During this process, the endometrium, the inner lining of the uterus, is shed and expelled through the vagina. This shedding prepares the uterus for a fresh cycle of potential implantation.
This process differs significantly from the estrous cycle, common in most other mammals. In an estrous cycle, the uterine lining is reabsorbed by the body if conception does not occur, rather than being shed externally. Menstruation involves visible external bleeding, distinguishing it from the estrous cycles observed in many non-primate mammals.
Variations Among Monkey Species
While menstruation is a shared characteristic among some primates, its presence and manifestation vary considerably across different monkey species. Old World monkeys, such as macaques, baboons, and chimpanzees, exhibit clear menstrual cycles physiologically similar to those of humans. Rhesus macaques, for example, typically have a menstrual cycle lasting approximately 28 days, with visible bleeding occurring over several days, though the volume may be less than in humans. This similarity makes them valuable models for studying human reproductive health.
Conversely, most New World monkeys, including marmosets, tamarins, and capuchins, generally do not display overt menstruation. These species primarily undergo estrous cycles. While some New World monkeys might exhibit very minimal or unnoticeable bleeding, it is not comparable to the distinct menstrual flow seen in Old World primates. The observable differences in cycle type and bleeding are a significant point of divergence in primate reproductive strategies.
Evolutionary Insights of Menstruation
The evolution of menstruation in some primate lineages, while absent in others, has led to several hypotheses. One prominent idea is the “energy conservation” theory, suggesting that shedding the uterine lining might be less energetically costly than maintaining it in a receptive state if conception does not occur, particularly if the lining is not perfectly suited for implantation. Another hypothesis, the “maternal-fetal conflict” theory, proposes that menstruation evolved as a mechanism for the mother to exert control over the quality of the embryo.
This theory suggests that by shedding the uterine lining, the mother can reject embryos that are not robust or healthy enough, preventing the allocation of resources to a potentially non-viable pregnancy. While menstruation is most pronounced in certain primates, it is also observed in a few other mammalian groups, such as some bats and elephant shrews, suggesting this reproductive strategy may have evolved independently multiple times. These theories highlight the complex interplay of reproductive biology and evolutionary pressures in different mammalian groups.
References
1. “The menstrual cycle of the rhesus monkey (Macaca mulatta) | PNAS”. Accessed July 25, 2025. [https://www.pnas.org/doi/10.1073/pnas.1203791109](https://www.pnas.org/doi/10.1073/pnas.1203791109)
2. “Why do women menstruate? | The BMJ”. Accessed July 25, 2025. [https://www.bmj.com/content/311/7017/1424.full](https://www.bmj.com/content/311/7017/1424.full)
3. “Menstruation: a maternal-fetal conflict perspective – PMC”. Accessed July 25, 2025. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3058880/](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3058880/)