Menstruation is a widely recognized biological process in human females, often assumed to be a standard feature across the mammalian class. This assumption is inaccurate, as the phenomenon of cyclical, visible bleeding is highly unusual in the animal kingdom. The vast majority of mammals follow a different reproductive pattern, making the shedding of the uterine lining a rare evolutionary event.
Defining True Menstruation
True menstruation is precisely defined as the periodic, spontaneous shedding of the functional layer of the endometrium, the tissue lining the uterus, when pregnancy does not occur. This process is triggered by the programmed decline of progesterone levels following ovulation and results in the visible discharge of blood and mucosal tissue through the vagina. The key distinction lies in the shedding of the thickened uterine lining rather than its reabsorption.
The reproductive cycle common to most placental mammals, such as dogs, cows, and rodents, is the estrous cycle, often referred to as “heat”. In these animals, the uterine lining is also prepared for pregnancy, but if fertilization does not take place, the tissue is reabsorbed by the body. This reabsorption, sometimes called covert menstruation, is metabolically efficient and results in little to no external bleeding.
While some non-menstruating animals, like dogs, may exhibit bloody discharge, this is typically a consequence of vascular changes during the estrous phase, not the shedding of a fully developed endometrium. Therefore, the simple presence of visible blood does not qualify as true menstruation.
The Exclusive Club: Animals That Menstruate
The list of mammals that experience true, overt menstruation is surprisingly short, making it an exclusive biological club. The largest and most well-known group is the higher primates, including humans, apes, and Old World monkeys such as baboons and macaques. Within this group, the length and heaviness of the bleed vary; for instance, non-human primates often have a lighter menstrual flow than humans.
Beyond primates, menstruation has been definitively observed in two other, highly disparate groups of mammals. The first is a small number of bat species, including the black mastiff bat and certain fruit bats, which have menstrual cycles lasting between 21 and 33 days. This suggests that menstruation has evolved independently in the bat lineage.
The second non-primate group is the elephant shrew, also known as the sengi, a small African mammal belonging to an ancient family. Although their menstruation in the wild is not always a continuous cyclical event, their uterine changes closely mirror the shedding of the endometrium seen in primates. The Cairo spiny mouse is a recently confirmed addition, being the only known rodent to spontaneously menstruate.
The Evolutionary Puzzle: Why is Menstruation So Rare?
The rarity of menstruation suggests that the evolutionary forces favoring it were highly specific. The leading theory links menstruation to a process called spontaneous decidualization. Decidualization is the transformation of the uterine lining into a specialized tissue, the decidua, which is essential for a successful pregnancy.
In most mammals, this transformation is triggered only after an embryo implants in the uterus. However, in menstruating species, this decidualization happens spontaneously in every cycle, regardless of whether an embryo is present. This pre-emptive preparation of the uterus for implantation means that if pregnancy fails, the fully transformed decidual lining must be shed, resulting in menstruation.
This spontaneous preparation is thought to be an adaptation related to the highly invasive nature of the embryo in these specific species. By pre-emptively building a highly responsive lining, the mother’s body gains a degree of control, allowing it to “test” the quality of the implanting embryo. A defective embryo might be rejected by the uterine lining, which is then shed in the menstrual flow.
The alternative hypothesis suggests that shedding the lining is a defense mechanism. This theory posits that menstruation serves to remove pathogens that may have entered the uterus, eliminating potential infections before a pregnancy is established. Regardless of the exact reason, the high metabolic cost of shedding and rebuilding the endometrium each cycle indicates a significant reproductive advantage must exist to justify this energy expenditure over the energy-saving method of reabsorption.