The menstrual cycle is a recurring physiological process that prepares the female reproductive system for a potential pregnancy. It involves the buildup of a specialized, highly vascularized uterine lining, known as the endometrium. If fertilization does not occur, the cycle concludes with the physical shedding of this lining. This mechanism of endometrial expulsion is biologically uncommon, even though reproductive cycles are universal across all mammalian species.
Understanding Menstruation Versus Estrus
The primary difference between the menstrual cycle and the estrous cycle lies in the fate of the endometrial lining when pregnancy does not happen. In the estrous cycle, common to the vast majority of mammals, the lining is completely reabsorbed by the body. This process, referred to as “covert cycling,” means there is no external bleeding. Non-primate mammals like dogs, cows, and rodents follow this pattern.
The menstrual cycle, in contrast, involves the expulsion of the endometrium, resulting in visible blood loss, termed “overt cycling.” This shedding is triggered when hormone levels, specifically progesterone, drop significantly after no implantation occurs. The highly specialized, thick lining prepared for a fertilized egg is broken down and expelled through the vagina. This expulsion distinguishes true menstruation from the estrous cycles of other animals.
The physiological complexity of the menstrual cycle is linked to spontaneous decidualization. This involves the uterus pre-emptively creating an aggressively prepared lining regardless of a signal from an embryo. Because this specialized lining is highly developed and vascularized, its maintenance is metabolically costly. Its disposal is necessary if it is not used, making the decision to build and then shed this lining a defining characteristic of true menstruation.
The Mammalian Groups That Menstruate
The phenomenon of true menstruation is remarkably rare, occurring in less than two percent of all mammalian species. The most widely recognized group that menstruates is the Catarrhini infraorder of primates, which includes humans, apes, and all Old World Monkeys. Species such as chimpanzees, gorillas, baboons, and macaques all exhibit a menstrual cycle.
Beyond the primate order, the trait appears sporadically in a few other, unrelated mammalian groups. Certain species of bats, for example, have been confirmed to menstruate. At least four species, including the black mastiff bat and Seba’s short-tailed bat, show evidence of this cycle, often with cycles lasting between 21 and 27 days.
Another small mammal exhibiting a menstrual cycle is the elephant shrew (Macroscelidea). These African insectivores are not closely related to primates or bats, suggesting that menstruation evolved independently in their lineage. Recent research has also identified the Cairo spiny mouse (Acomys cahirinus) as a rodent species that experiences a menstrual cycle, adding to the short list of non-primate menstruating mammals.
Evolutionary Theories for Menstruation
The scarcity of menstruation across the mammalian class has led scientists to develop several theories explaining why this seemingly costly process evolved. One prominent hypothesis is that menstruation functions as a defense mechanism against pathogens. The theory suggests that the periodic shedding of the endometrium helps to flush out infectious agents, such as bacteria or viruses, that may have been introduced to the uterus. This mechanism would be particularly beneficial in species that engage in copulation throughout their cycle, rather than only during a restricted fertile window.
Another major theory centers on the concept of energy conservation and metabolic efficiency. Building and maintaining a thick, highly vascularized uterine lining requires a significant investment of a female’s resources. It may be metabolically cheaper to dismantle and expel the complex lining when it goes unused than to keep that specialized tissue maintained indefinitely, awaiting a potential pregnancy. The temporary inconvenience and resource loss of a period is considered a lower cost than the perpetual maintenance of an “alert” uterus.
A third, more recent theory links menstruation to the high degree of invasiveness exhibited by the embryo during implantation in these specific species. Menstruating species undergo spontaneous decidualization, which aggressively prepares the uterine lining before the embryo arrives. This pre-emptive preparation is thought to allow the mother to screen or vet the quality of the incoming embryo. By developing a highly reactive lining, the female’s body retains control, potentially rejecting a defective or non-viable embryo before it can draw too heavily on maternal resources.