Most animals do have reproductive cycles, but the vast majority quietly reabsorb their uterine lining rather than shedding it as visible bleeding. Menstruation, the external shedding of the uterine lining, is genuinely rare in the animal kingdom. Outside of humans, only a handful of primates, a few bat species, elephant shrews, and one recently discovered rodent are known to menstruate. The real question is not so much why other animals skip periods but why a small club of species, humans included, evolved this seemingly wasteful process in the first place.
What Makes Menstruation Different From an Estrous Cycle
Almost every female mammal goes through a reproductive cycle. In the vast majority of species, this takes the form of an estrous cycle. During estrus, the uterine lining thickens in preparation for a potential pregnancy. If no embryo implants, the body simply reabsorbs that tissue back into itself. No blood, no mess, no external sign that the cycle happened at all. The animal may show behavioral changes during the fertile window (the “heat” period that gives the estrous cycle its name), but the end of the cycle passes invisibly.
In menstruating species, things work differently. Instead of reabsorbing the lining, the outer layers of the endometrium break down, detach, and exit through the cervix along with blood from ruptured vessels. This is driven by a sharp drop in progesterone levels after ovulation when no pregnancy has occurred. The distinction matters because it points to a fundamentally different way of preparing the uterus: menstruating species build up a thicker, more elaborately remodeled lining that cannot simply be tidied away when it is no longer needed.
Which Animals Actually Menstruate
The list is short and eclectic. Among primates, menstruation is found in humans, great apes, Old World monkeys, and some New World monkeys. Natural menstruation outside primates is rare, limited to certain bat species, elephant shrews, and the Egyptian spiny mouse.
The bat evidence is particularly well documented. Short-tailed fruit bats shed necrotic portions of the endometrium’s superficial layer along with associated bleeding when they have a regressing corpus luteum and no pregnancy, a process researchers have confirmed as true menstruation rather than some other form of uterine bleeding.1PubMed. Menstruation in short-tailed fruit bats (Carollia spp.) Fulvous fruit bats show an even more striking parallel to humans. Their cycle lasts about 33 days, with vaginal bleeding on a single day. Hormone patterns, including a preovulatory surge in pituitary hormones and a late-cycle crash in progesterone that triggers bleeding, closely mirror the human menstrual cycle.2Biology of Reproduction. Wild Fulvous Fruit Bats (Rousettus leschenaulti) Exhibit Human-Like Menstrual Cycle
The Egyptian spiny mouse was only confirmed as a menstruating species in 2016, making it the first known menstruating rodent. Researchers found that these mice have an average cycle length of about nine days, with visible blood cells appearing over roughly three of those days. Their endometrium thickens during the luteal phase and then sheds at the end of each infertile cycle, just as in humans.3PubMed. First evidence of a menstruating rodent: the spiny mouse (Acomys cahirinus) The discovery was significant because standard laboratory mice do not menstruate, which has made studying menstrual disorders in animal models surprisingly difficult. The spiny mouse now offers researchers a small, accessible species in which to study the process outside primates.4PubMed Central. The Spiny Mouse-A Menstruating Rodent to Build a Bridge From Bench to Bedside
Detailed tissue studies of the spiny mouse show that the shedding process looks remarkably human-like: decidualized endometrial tissue containing spiral arterioles breaks down, large masses of tissue slough off in discrete regions over roughly 24 to 36 hours, and by 48 to 72 hours the luminal epithelium has already regenerated.5Human Reproduction. Characterization of human-like menstruation in the spiny mouse: comparative studies with the human and induced mouse model This rapid breakdown-then-rebuild sequence is a hallmark of menstruation across every species that does it.
Spontaneous Decidualization Is the Key
The reason menstruation happens in some species and not others comes down to how the uterine lining prepares for pregnancy. In most mammals, the endometrium only undergoes decidualization, a dramatic transformation of its stromal cells into a thick, nutrient-rich tissue, in response to a signal from an implanting embryo. No embryo, no decidualization, no thick lining to deal with afterward. The body simply reabsorbs whatever modest thickening occurred.
In humans and the other menstruating species, decidualization happens spontaneously. It is triggered by the postovulatory rise in progesterone, not by the embryo.6Endocrine Reviews. Cyclic Decidualization of the Human Endometrium in Reproductive Health and Failure The endometrium transforms every cycle regardless of whether fertilization has occurred. This creates a thicker, more metabolically active lining with specialized blood vessels. When progesterone drops at the end of an infertile cycle, that heavily remodeled tissue cannot simply be reabsorbed. Instead, it breaks down and is expelled. Menstruation is essentially the cleanup cost of preparing the uterus for pregnancy before knowing whether pregnancy will happen.
Research on mouse models shows there is a critical window for this process. If progesterone is restored within about 12 hours of the initial drop, bleeding and shedding can be completely blocked. But by 16 hours, the tissue breakdown has progressed past the point of no return, and the shedding proceeds regardless of whether progesterone is added back.7Human Reproduction. A critical period of progesterone withdrawal precedes endometrial breakdown and shedding in mouse menstrual-like model This narrow window helps explain why menstruation, once set in motion each cycle, follows through to completion.
Why Would Evolution Favor Spontaneous Decidualization
If menstruation costs the body resources and energy, why would natural selection favor a system that prepares the uterus before the embryo even arrives? Researchers have proposed several explanations, and the honest answer is that the debate is not fully settled. Multiple mechanisms may be operating at once.
One prominent hypothesis centers on maternal-fetal conflict. In species with highly invasive placentas, like humans and great apes, the embryo burrows aggressively into the uterine wall to establish a blood supply. This invasiveness benefits the embryo but can be dangerous for the mother if the pregnancy goes wrong. Spontaneous decidualization may have evolved as a maternal defense: by pre-transforming the endometrium, the mother’s tissue is already structured to manage and contain embryonic invasion from the moment of implantation. This hypothesis argues that spontaneous decidualization evolved through a gradual shift in which the decidualization response, originally triggered by the embryo itself, was co-opted to occur before implantation.8PubMed Central. The evolution of menstruation: a new model for genetic assimilation
A second, related idea is that spontaneous decidualization allows the endometrium to screen embryos for quality before fully committing to pregnancy. Human embryos have an unusually high rate of chromosomal abnormalities compared to other mammals. Research has shown that decidualized endometrial stromal cells can sense embryo quality at the time of implantation. Cyclical decidualization followed by menstrual shedding may act as a filter, limiting how much the mother invests in a developmentally impaired embryo.9PubMed Central. Natural selection of human embryos: decidualizing endometrial stromal cells serve as sensors of embryo quality upon implantation When this screening process is impaired and the endometrium lets too many low-quality embryos implant, the result can be recurrent pregnancy loss: the pregnancy begins but fails shortly after.10PLoS ONE. Natural Selection of Human Embryos: Impaired Decidualization of Endometrium Disables Embryo-Maternal Interactions and Causes Recurrent Pregnancy Loss
A third hypothesis focuses on energy conservation. Maintaining the endometrium in a fully decidualized, metabolically active state is expensive. In the regressed state after shedding, oxygen consumption in human endometrial tissue drops by roughly sevenfold. Across the whole body, metabolic rate runs at least 7% lower during the follicular phase (the rebuilding phase after a period) than during the luteal phase, which adds up to a meaningful energy savings over multiple cycles.11PubMed. The evolution of endometrial cycles and menstruation Under this view, shedding and rebuilding the lining each cycle is actually cheaper than keeping it primed and waiting month after month. The cycle revs up and revs down, conserving energy in between.
An older hypothesis, proposed in the early 1990s, suggested that menstruation functions as a defense against sperm-borne pathogens, with menstrual blood exerting mechanical pressure on uterine tissue and delivering immune cells throughout the uterine cavity.12PubMed. Menstruation as a defense against pathogens transported by sperm This idea has largely fallen out of favor because species that do not menstruate seem to handle uterine pathogens just fine, and the timing of menstrual shedding does not align well with the window of greatest infection risk. It remains a historical footnote in the debate rather than a leading explanation.
Scarless Healing and What Makes the Endometrium Unusual
One of the more remarkable features of menstruation is what happens after the bleeding stops. The endometrium regenerates rapidly and completely, without forming scar tissue. Over a reproductive lifetime, this process repeats up to 400 times, and each time the tissue heals as if nothing happened.13PubMed Central. Scar-Free Healing of Endometrium: Tissue-Specific Program of Stromal Cells and Its Induction by Soluble Factors Produced After Damage Compare this to skin, where even a minor cut can leave a mark. The endometrium is essentially the only tissue in the adult human body that can undergo repeated injury and heal without any scarring.
Research into how this works has found that during endometrial healing, soluble factors produced after tissue damage actively prevent stromal cells from converting into myofibroblasts, the cell type responsible for scar formation in skin and other tissues. Without those factors, endometrial stromal cells would scar just like cells from any other tissue. But the endometrial environment specifically suppresses that pathway and instead organizes the rebuilding of the tissue’s structural framework in a way that avoids bulky connective tissue.13PubMed Central. Scar-Free Healing of Endometrium: Tissue-Specific Program of Stromal Cells and Its Induction by Soluble Factors Produced After Damage Multiple immune cell populations also play a role in orchestrating this repair.14PubMed Central. Mechanisms of Scarless Repair at Time of Menstruation: Insights From Mouse Models
Understanding this scarless healing is a goal in its own right, well beyond reproductive health. If researchers could replicate the endometrium’s trick in other tissues, it could transform treatment for wounds, surgical incisions, and conditions like uterine scarring from cesarean sections. The spiny mouse’s emergence as a lab model has opened new avenues for this work, since previously the only way to study menstrual-like shedding in rodents was through artificial hormone manipulation in standard mice.
How the Human Cycle Compares
The textbook “28-day cycle” is more of a rough average than a fixed rule. A large study tracking over 1.5 million women through a mobile phone app found that only about 16% actually had a median cycle length of exactly 28 days. A 27-day or 29-day cycle was nearly as common, each accounting for roughly 12% of participants.15PubMed Central. Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study Cycle length varied considerably with age, stress, and physical activity levels. Women with shorter cycles were more likely to report high stress and less physical activity, and also tended to have lighter menstrual flow.
For comparison, the fulvous fruit bat’s cycle runs about 33 days with a single day of bleeding, while the spiny mouse cycles through in just under nine days with about three days of visible blood. What all menstruating species share is not the timing but the underlying hormonal architecture: a progesterone-driven buildup of decidualized tissue, followed by a progesterone crash that triggers shedding. The details of duration and flow volume are tuned to each species’ reproductive strategy and body size.
Lifetime Menstruation and Its Health Consequences
One way in which modern human women differ from both their ancestors and other menstruating species is the sheer number of cycles they experience. In hunter-gatherer and pre-industrial populations, women spent much of their reproductive years either pregnant or lactating, both of which suppress ovulation and menstruation. Today, with smaller families and shorter breastfeeding durations, it is common for a woman in an industrialized country to go through 400 or more cycles over a lifetime.
This high number of cycles has health implications. Each cycle exposes the endometrium to estrogen-driven growth followed by progesterone-driven transformation. A large Norwegian study found that each additional year of menstruation was associated with about a 9% higher risk of endometrial cancer. Women with 40 or more lifetime years of menstruation had substantially higher risk than those with fewer than 25 years, and this relationship held even after accounting for factors like hormone therapy, diabetes, and body mass index.16PubMed Central. Lifetime number of years of menstruation as a risk index for postmenopausal endometrial cancer in the Norwegian Women and Cancer Study The researchers estimated that roughly two-thirds of endometrial cancer cases in the study population were attributable to having 25 or more years of menstruation.
This does not mean menstruation itself causes cancer. It means that the repeated hormonal stimulation of endometrial tissue, which menstruation reflects, accumulates as a risk factor over time. Pregnancy, breastfeeding, and hormonal contraceptives that suppress ovulation all reduce the total number of cycles and are associated with lower endometrial cancer risk. The modern pattern of many consecutive uninterrupted cycles is, in evolutionary terms, quite new, and the human body did not evolve to handle it at this scale.
Menopause and the End of Cycling
Menstruation and menopause are distinct phenomena, but they intersect in interesting ways. Menopause, the permanent end of reproductive cycling, is itself unusual among mammals. Toothed whales are the only other mammal group in which menopause has evolved multiple times independently, making them a valuable point of comparison for understanding why reproductive cessation would be favored by natural selection.17PubMed Central. The evolution of menopause in toothed whales Analyses of species data suggest that menopause requires a combination of long lifespan, group living, and a substantial gap between female and male average lifespans.18PubMed. How evolutionary biology can explain why human and a few marine mammal females are the only ones that are menopausal
The whale comparison is illuminating because while toothed whales evolved menopause, they did not evolve menstruation. Whales have estrous cycles with endometrial reabsorption, not shedding. This underscores that menstruation and menopause are driven by entirely separate evolutionary pressures. Menstruation is tied to how invasively the embryo implants and how the uterus manages that invasion. Menopause appears to be about social structure, intergenerational cooperation, and the costs of late-life reproduction. A species can have one without the other, and most do.
Why Standard Lab Mice Do Not Menstruate
Given how much biomedical research relies on mice, it is worth noting that standard laboratory mice undergo a four- to five-day estrous cycle with complete reabsorption of the endometrium. Their endometrial lining does not decidualize spontaneously; it only does so in response to an implanting embryo. This has been a persistent obstacle for researchers studying menstrual disorders like endometriosis, heavy menstrual bleeding, and adenomyosis. For decades, the only option was to artificially induce a menstruation-like process in mice by first priming them with hormones and then withdrawing progesterone, creating a model that approximates but does not replicate natural menstruation.19PubMed. A critical period of progesterone withdrawal precedes endometrial breakdown and shedding in mouse menstrual-like model
The discovery of natural menstruation in the spiny mouse changed the landscape of this research. Here was a small, manageable rodent that menstruated on its own, with a short cycle, and whose endometrial tissue showed the same decidualization, shedding, and regeneration sequence seen in humans.20PubMed Central. Monkeys, mice and menses: the bloody anomaly of the spiny mouse Having a natural rodent model matters because artificially induced processes do not always capture the full biology. Immune responses, tissue remodeling, and repair mechanisms may differ when the cycle is engineered versus when it occurs spontaneously. The spiny mouse is still a relatively new research subject, and colonies are not yet as widely established as standard mouse strains, but it is increasingly being used to study everything from endometrial repair to the early events of implantation.