The menstrual cycle is a reproductive process, typically lasting about 28 days, involving the preparation of the uterine lining (endometrium) for a potential fertilized egg. If conception does not occur, the thickened lining breaks down in a process called menstruation. This cyclical, spontaneous shedding of the endometrium is the physiological benchmark against which other species are measured. This expulsion of tissue and blood is actually a rare reproductive strategy among mammals.
The Mammalian Default: The Estrous Cycle
Menstruation is the biological exception rather than the rule among mammals. The reproductive cycle used by the majority of mammals, including most livestock, rodents, and carnivores, is the estrous cycle. If pregnancy does not occur, this cycle culminates in the reabsorption of the uterine lining instead of shedding it. By recycling the endometrial tissue, the female body conserves energy and nutrients, leaving no external sign of a reproductive cycle.
The estrous cycle includes a distinct period of heightened sexual receptivity known as estrus, or “heat,” often accompanied by clear behavioral and physical signals to attract a mate. In non-primate mammals like dogs, bloody discharge can occur during the proestrus phase, which is sometimes mistaken for menstruation. This bleeding is caused by the dilation of blood vessels due to rising estrogen levels, and it does not represent the spontaneous breakdown of the uterine lining. In estrous cycles, the endometrium is reabsorbed, while in menstrual cycles, it is actively expelled.
Species That Menstruate
True menstruation is an uncommon trait, found in less than 2% of the approximately 5,400 known mammal species. The vast majority of species that menstruate belong to the order Primates, specifically the higher primates, including Old World monkeys, apes, and humans. Great apes, such as chimpanzees and gorillas, have menstrual cycles that range from 29 to 35 days, closely mirroring the human pattern.
The trait is also found in a few non-primate lineages, suggesting it evolved independently multiple times. Certain species of bats, notably the black mastiff bat and the wild fulvous fruit bat, exhibit a menstrual cycle. Another menstruating mammal is the elephant shrew, a small insectivore from Africa, which has a cycle length of around 28 days. Most recently, the Cairo spiny mouse was identified as a menstruating rodent.
Evolutionary Explanations for Endometrial Shedding
Shedding tissue and blood appears to be energetically wasteful compared to the reabsorption seen in estrous cycles. Two primary hypotheses explain the persistence of this costly mechanism. The leading theory is the Decidualization Hypothesis, which links menstruation to the unique way the uterus prepares for implantation. In menstruating species, the endometrium undergoes spontaneous decidualization, preparing the lining for a potential embryo before it even arrives.
This preparation provides a form of quality control, allowing the mother to screen and reject genetically defective embryos early on. The decidualized tissue has a limited lifespan, making it difficult to maintain or reabsorb if no pregnancy occurs. Therefore, shedding the lining may be the safer, more efficient way to dispose of the tissue.
A second, older theory is the Pathogen Protection Hypothesis, which suggests that menstruation serves to cleanse the uterus of potential infections. This idea proposes that the periodic flushing action helps to remove sperm-borne pathogens introduced during copulation. While this hypothesis has been debated, the current consensus leans toward the decidualization process as the main driver. The evolution of menstruation appears to be a consequence of managing a deep, invasive placental attachment.