Menstruation is a physiological process that often feels universal, yet it is surprisingly rare across the vast class of Mammalia. It is confined to a tiny fraction of all mammalian species. The vast majority of mammals manage their reproductive cycles through a fundamentally different biological mechanism. Understanding this disparity highlights a deep evolutionary split in how different groups of animals prepare the uterus for potential pregnancy.
Defining the Menstrual Cycle
The menstrual cycle is a coordinated series of hormonal and structural changes that prepare the uterus for the possibility of supporting a fertilized egg. The endometrium, the thick, blood-rich inner lining of the uterus, proliferates and becomes enriched with blood vessels to create an ideal environment for an embryo to implant and grow.
If fertilization does not occur, the hormonal signals that maintained the thickened lining drop dramatically. This change triggers the breakdown of the functional layer of the endometrium. The subsequent expulsion of this tissue, blood, and mucus through the vagina is what is medically defined as menstruation.
This shedding process is a complete dismantling and removal of the prepared uterine lining. It differs significantly from other forms of reproductive bleeding that might occur in non-menstruating mammals.
The Mammalian Taxa That Menstruate
The phenomenon of true, external menstruation is limited to a small and scattered collection of mammals, representing less than 2% of all species. The most recognized group that menstruates includes humans, apes, and most Old World and New World monkeys, which fall under the category of primates. This makes the Primate order the largest group in which the menstrual cycle is common.
Beyond primates, the list of menstruating mammals is short and surprising, indicating that this trait evolved independently in unrelated lineages. Certain species of bats, such as the black mastiff bat and the wild fulvous fruit bat, have been observed to exhibit a menstrual cycle. The elephant shrew, a small African mammal that is not a true shrew, also menstruates.
The Cairo spiny mouse, a rodent native to North Africa, is another confirmed non-primate species to experience true menstruation. The patchy distribution of this trait suggests that it arose through at least four separate evolutionary events across the mammalian family tree.
The Majority Strategy: Endometrial Reabsorption
The reproductive strategy utilized by over 98% of mammalian species is known as the estrous cycle, which operates without the external shedding of the uterine lining. Mammals like dogs, cats, cows, horses, and rodents all follow this cycle, which is characterized by a phase called “estrus,” or “heat.” During this time, the female is fertile and receptive to mating.
Similar to the menstrual cycle, the estrous cycle involves the uterine lining thickening in preparation for pregnancy. However, if fertilization does not happen, the body does not expel the excess tissue. Instead, the endometrium is completely reabsorbed by the body through a process called endometrial reabsorption.
This reabsorption is metabolically efficient, as the body conserves the nutrients, blood, and tissue components of the lining rather than discarding them. While some non-menstruating mammals, such as dogs, may exhibit bloody vaginal discharge during the estrus phase, this is not true menstruation. This spotting is due to blood leaking from the highly vascularized reproductive tract and does not represent the cyclical breakdown and shedding of the entire functional layer of the endometrium.
Evolutionary Theories Behind Menstruation
The evolution of menstruation poses a scientific puzzle because shedding tissue and blood appears to be energetically costly compared to reabsorption. Scientists have developed several hypotheses to explain the evolutionary advantage of this seemingly wasteful process. One prominent idea is the Energy Conservation Hypothesis, which suggests that in menstruating species, constantly maintaining a metabolically demanding uterine lining is more expensive than periodically building and then shedding it.
This theory posits that the cyclical destruction and renewal of the endometrium is an energy-saving trade-off. It is argued that the energy required to rebuild the lining is less than the energy needed to keep it in a state of readiness for extended periods without pregnancy. This mechanism allows the body to conserve resources during non-fertile periods.
Another major hypothesis is the Protective Hypothesis, which frames menstruation as a defense mechanism. This theory suggests that the external shedding of the uterine lining is a way to remove pathogens that may have entered the uterus during the reproductive phase. The menstrual flow effectively flushes out bacteria and other foreign agents, helping to protect the female reproductive tract from infection.