All great apes menstruate, and so do their smaller relatives, the gibbons. Chimpanzees, bonobos, gorillas, and orangutans each go through a monthly hormonal cycle that, when no pregnancy occurs, ends with shedding of the uterine lining and vaginal bleeding. The process is driven by the same progesterone-withdrawal mechanism behind human periods, and cycle lengths across the ape family fall remarkably close to our own. What gets more interesting is who else in the animal kingdom shares this trait and why evolution landed on it at all.
Which Great Apes Have Periods
All four genera of great apes experience menstrual cycles. Chimpanzees and bonobos have cycles averaging roughly 35 to 36 days, somewhat longer than the textbook 28-day human cycle but following the same hormonal arc. Gorillas cycle on a similar schedule, typically around 30 to 34 days. Orangutans, studied extensively in captivity, show a mean cycle length of about 30 days, though individual females vary quite a bit from cycle to cycle.1PubMed. A composite menstrual cycle of captive orangutans, with associated hormonal and behavioral variability
The amount of visible bleeding differs across species. Human menstruation tends to be relatively heavy and prolonged compared to other apes. Chimpanzees and bonobos sometimes show obvious external bleeding, but gorillas and orangutans often menstruate with so little external evidence that caretakers or field researchers can miss it entirely. That does not mean it is not happening; tissue shedding and low-level bleeding still occur internally. Researchers monitoring captive apes typically detect cycles through hormonal assays or by collecting vaginal swabs rather than waiting for visible blood.
One reason chimpanzee and bonobo menstruation is easier to spot has to do with their prominent anogenital swellings. These swellings balloon up around ovulation and deflate afterward, giving researchers a visible marker for where each female stands in her cycle. When the swelling fully subsides and a new cycle begins, external blood may be noticeable for a day or two. Gorillas and orangutans lack such dramatic swellings, so their cycles are far less conspicuous to an observer.
Gibbons and Other Lesser Apes
Gibbons, sometimes called lesser apes, also menstruate. A long-running study of captive Javan gibbons tracked 18 years of reproductive data from 11 females and found a menstrual bleeding interval averaging about 26 days, with sexual swellings near ovulation giving a slightly longer cycle estimate of about 27 days.2PubMed. Female reproductive parameters in the Javan gibbon (Hylobates moloch) Those numbers line up closely with other gibbon species that have been studied hormonally. Gibbons tend to show clearer external menstrual bleeding than gorillas or orangutans, making their cycles easier to track without blood draws.
This means that menstruation spans the entire ape superfamily. From the largest (gorillas) to the smallest (gibbons weighing as little as five or six kilograms), every ape species studied to date shows cyclic endometrial shedding driven by hormonal fluctuations. Humans are not unusual among apes for menstruating; if anything, we stand out for how heavy and prolonged our bleeding is relative to body size.
Monkeys That Menstruate Too
The trait is not exclusive to apes. Old World monkeys, including macaques, baboons, and mandrills, are well-documented menstruators. Rhesus macaques, in fact, have been laboratory models for menstrual research for decades. Their cycles run about 28 days, close enough to the human average that they are frequently used to study reproductive biology and contraceptive development.
New World monkeys present a more varied picture. Many species in this group do menstruate, though the bleeding can be so faint that it is described as “weakly evident,” particularly in the callitrichid family, which includes marmosets and tamarins.3International Zoo Yearbook. Reproductive characteristics of New World monkeys Capuchins and spider monkeys, by contrast, have more recognizable menstrual bleeding. The overall pattern is that menstruation appears across the entire primate order, but its visibility and volume vary enormously.
Animals Outside the Primate Family That Menstruate
One of the most surprising findings in reproductive biology is that a handful of non-primate mammals also menstruate. The list is short but diverse, and it challenges the old assumption that periods are a “primate thing.”
At least five species of bats are known to shed their uterine lining cyclically. The short-tailed fruit bat shows a true menstrual process, with necrotic portions of the endometrium sloughing off along with associated bleeding at the end of infertile cycles.4Journal of Reproduction and Fertility. Menstruation in short-tailed fruit bats (Carollia spp.) The fulvous fruit bat goes even further: its 33-day menstrual cycle mirrors the hormonal arc of a human cycle so closely, with a progesterone crash preceding bleeding and similar endometrial changes throughout, that researchers have proposed it as a non-primate laboratory model for studying menstrual disorders.5Biology of Reproduction. Wild Fulvous Fruit Bats (Rousettus leschenaulti) Exhibit Human-Like Menstrual Cycle Other bat species in which menstruation has been documented include the long-tongued bat, the black mastiff bat, and a piscivorous vespertilionid bat.6Biology of Reproduction. Characteristics of the endometrium in menstruating species: lessons learned from the animal kingdom
Elephant shrews, small insectivorous mammals native to Africa, also menstruate. Their infertile cycles end with the shedding of a glandular outgrowth from the uterine stroma in a process that researchers describe as equivalent to menstruation.7PubMed. Classics revisited: C. J. van der Horst on pregnancy and menstruation in elephant shrews This was documented decades ago but received relatively little attention, partly because elephant shrews are not common research animals.
The most recent addition to the list is the Egyptian spiny mouse, discovered to menstruate only in 2016. This was a genuine surprise because rodents were long considered textbook examples of non-menstruating mammals. The spiny mouse has a short cycle of about nine days, with visible red blood cells in vaginal samples over roughly three days.8PubMed. First evidence of a menstruating rodent: the spiny mouse (Acomys cahirinus) Follow-up work confirmed that the endometrial breakdown resembles human menstrual shedding, with focal epithelial breakdown and similar inflammatory markers detected in the menstrual fluid.9Human Reproduction. Characterization of human-like menstruation in the spiny mouse: comparative studies with the human and induced mouse model Because spiny mice are small, breed readily in captivity, and have short cycles, they are now being developed as a laboratory model for research into conditions like endometriosis and abnormal uterine bleeding.10PubMed Central. The Spiny Mouse-A Menstruating Rodent to Build a Bridge From Bench to Bedside
What Triggers the Bleeding
The immediate cause of menstruation in all these species is the same: a drop in progesterone. After ovulation, the corpus luteum in the ovary produces progesterone, which maintains and thickens the uterine lining. If no embryo implants, the corpus luteum breaks down and progesterone levels plummet. In macaques, research has pinpointed a critical window of about 36 hours of progesterone withdrawal that commits the endometrium to shedding; once that window passes, menstruation will proceed even if you experimentally intervene.11PubMed Central. A critical period of progesterone withdrawal precedes menstruation in macaques
The progesterone drop unleashes a cascade of tissue-breakdown enzymes called matrix metalloproteinases. These enzymes are held in check throughout the cycle by their natural inhibitors, but when progesterone withdraws, the balance tips. The enzymes begin degrading the upper layers of the endometrium. Immune cells, including neutrophils and macrophages, flood the tissue in a process that closely resembles inflammation.12PubMed. Menstruation: induction by matrix metalloproteinases and inflammatory cells The tissue-breakdown enzymes and the inflammatory response together produce the bleeding and shedding that characterize menstruation.13Human Reproduction. Matrix metalloproteinases in normal menstruation
Progesterone withdrawal also triggers changes in blood-vessel biology within the endometrium. Research in both humans and macaques has shown that receptors for vascular growth factors get dramatically upregulated in the superficial stromal cells just before menstruation begins, an event that appears to interact with the tissue-breakdown enzymes to coordinate the shedding process.14The Journal of Clinical Endocrinology & Metabolism. Progesterone Withdrawal Up-Regulates Vascular Endothelial Growth Factor Receptor Type 2 in the Superficial Zone Stroma of the Human and Macaque Endometrium: Potential Relevance to Menstruation The fact that macaques and humans share these molecular details down to the same receptor types underscores how conserved this process is among menstruating primates.
Why Menstruation Evolved
If you think about it from a pure efficiency standpoint, menstruation seems wasteful. The body builds up a nutrient-rich uterine lining and then throws most of it away every month. Several competing hypotheses have tried to explain why natural selection would favor this.
One early idea proposed that menstruation evolved to flush sperm-borne pathogens from the uterus. It was a plausible-sounding hypothesis, but when its specific predictions were tested against the evidence, they did not hold up. Uterine pathogens are not reliably more prevalent before menses than after, and the heaviness of menstrual bleeding across primates does not track with mating-system promiscuity as the hypothesis predicted.15PubMed. The evolution of endometrial cycles and menstruation
The hypothesis that has gained the most traction centers on something called spontaneous decidualization. In menstruating species, the uterine lining begins transforming itself in preparation for a potential embryo before any embryo actually arrives. This transformation happens on its own, driven by the mother’s hormones, rather than being triggered by the embryo as it is in most non-menstruating mammals. The leading interpretation is that spontaneous decidualization evolved as a maternal defense mechanism. Embryos from species with aggressive, deeply invasive placentas pose a real metabolic and immunological cost to the mother. By pre-transforming the lining, the mother gains a degree of control over which embryos can successfully implant, essentially screening for viability before committing resources.16PubMed Central. The evolution of menstruation: a new model for genetic assimilation Menstruation, in this framework, is not the adaptive trait itself but a side effect. Once the lining decidualizes spontaneously and no pregnancy occurs, the transformed tissue has to go somewhere, and shedding it is the result.17PubMed. The significance and evolution of menstruation
This model also helps explain the scattered distribution of menstruation across the mammalian tree. Bats with invasive placentation menstruate. Elephant shrews with invasive placentation menstruate. The common thread is not being a primate; it is the degree to which the embryo invades maternal tissue. Where embryonic invasion is aggressive, the mother’s body appears to have evolved preemptive control of the lining, and menstruation follows.
Menopause in Apes
For a long time, menopause was considered essentially unique to humans (and possibly a few whale species). The thinking was that other primates simply kept cycling, at reduced rates, until they died. Recent work has complicated that picture considerably.
In captivity, chimpanzees undergo menopause near age 50, with hormonal signatures that match the human pattern: rising levels of gonadotropins (the pituitary hormones that try to stimulate the ovaries) and cessation of menstrual cycling.18PubMed Central. Menopause occurs late in life in the captive chimpanzee (Pan troglodytes) The key difference is that captive chimps can live into their late 50s or 60s, giving them a few post-reproductive years, whereas wild chimps rarely survive much past their mid-40s. So in the wild, most female chimpanzees die before menopause becomes relevant.
A landmark 2023 study changed the conversation by documenting genuine post-reproductive life spans in wild chimpanzees at Ngogo in Uganda’s Kibale National Park. Females who reached adulthood could expect to spend roughly one-fifth of their adult years in a post-reproductive state, complete with sharply rising gonadotropins after age 50.19PubMed Central. Demographic and hormonal evidence for menopause in wild chimpanzees That post-reproductive proportion is about half what you see in human hunter-gatherer populations, but it is unambiguously there. The finding suggests that menopause is not a human invention but rather an ancestral trait that humans have extended.
Evidence from other primate species broadly supports the view that reproductive senescence and eventual cessation of menstrual cycles occur across multiple primate lineages, though few non-human species routinely live long enough for it to be demographically significant.20PubMed Central. Menopause in nonhuman primates?
Endometriosis and Other Shared Reproductive Disorders
Any animal that menstruates can, in principle, develop the same menstrual-related pathologies that affect humans. This is not just theoretical. Primates in captivity spontaneously develop endometriosis, the condition in which tissue resembling the uterine lining grows outside the uterus. The lesions are morphologically identical to those in humans and appear at similar anatomical sites, though most non-human primates tend to develop milder forms of the disease.21ILAR Journal. Animal Studies in Endometriosis: A Review
Chimpanzees have also been documented with adenomyosis, a condition where endometrial tissue invades the muscular wall of the uterus itself. In baboons and macaques, adenomyosis has been linked to infertility and frequently co-occurs with endometriosis, much as it does in women.22Human Reproduction. Spontaneous adenomyosis in the chimpanzee (Pan troglodytes): a first report and review of the primate literature These findings have practical value beyond academic curiosity. Baboons and macaques with naturally occurring endometriosis are used as models to test potential therapies, and the spiny mouse is being explored for the same purpose now that its menstrual cycle has been confirmed.
The Cycle Synchrony Question
A persistent popular belief holds that females living in close proximity will synchronize their menstrual cycles. This idea, rooted in a 1971 study of human college roommates, has been extended in popular imagination to other primates. The evidence does not support it.
A rigorous study of semi-free-ranging mandrills used long-term cycle records and randomization analysis across ten group-years. The researchers found virtually no evidence of cycle synchrony. In the one year where a statistical signal initially appeared, it vanished once the analysis was corrected for multiple testing. Females within the same family lineage, who spent the most time together, were no more likely to cycle in sync than unrelated females.23PubMed. Do non-human primates synchronise their menstrual cycles? A test in mandrills The researchers noted that mandrills tend to conceive quickly enough that most females simply do not have enough consecutive non-pregnant cycles for synchronization to emerge, even if the mechanism existed.
Reanalysis of earlier claims of synchrony in chimpanzees and golden lion tamarins reached similar conclusions. Computer simulations demonstrated that natural variability in cycle length can produce patterns that look like synchrony when measured with the standard statistical approach, even when cycles are completely independent. The apparent synchrony in both species turned out to be a methodological artifact.24Behavioural Processes. Measurement and cycle variability: reexamining the case for ovarian-cycle synchrony in primates The consensus in the field today is that menstrual synchrony has not been convincingly demonstrated in any non-human primate.