Menstruation describes the monthly process where the human uterus sheds its inner lining, the endometrium, when a fertilized egg does not implant. This shedding involves the discharge of blood and tissue through the vagina. The question of whether other animals undergo a similar process is more complex than often assumed.
Understanding Mammalian Reproductive Cycles
Most female mammals experience reproductive cycles that prepare their bodies for potential pregnancy. These mechanisms vary significantly, primarily between estrous and menstrual cycles.
In an estrous cycle, the uterine lining thickens for pregnancy, but if conception does not occur, the body typically reabsorbs it. This means there is no external bleeding or discharge. Animals in estrus, often called “in heat,” display visible signs of fertility and receptivity to mating. This cycle ensures resources are not wasted if pregnancy does not occur.
The menstrual cycle, in contrast, involves the shedding of the uterine endometrium with distinct bleeding when pregnancy does not happen. Unlike estrous cycles, where females are typically only receptive during estrus, females with menstrual cycles can be receptive throughout their cycle. Menstrual cycles are much less common across the mammalian kingdom compared to estrous cycles.
Animals with True Menstruation
True menstruation, involving the regular shedding of the uterine lining with external bleeding, is rare among mammals, occurring in less than 2% of species. The most recognized group that menstruates, besides humans, is primates. This includes great apes like chimpanzees and bonobos, and Old World and New World monkeys. Not all primates menstruate; for example, lemurs do not, and some, like tarsiers, show only a weakly present cycle.
Beyond primates, a few other distinct mammalian groups also exhibit true menstruation. These include certain bat species, such as Seba’s short-tailed bat and Pallas’s long-tongued bat, with cycle lengths ranging from 21 to 27 days. Elephant shrews, small insectivorous mammals, also menstruate, though their cycle might be influenced by breeding seasons. The spiny mouse (Acomys cahirinus) is another rodent species known to have a menstrual cycle. The independent evolution of menstruation in these diverse groups suggests it arose multiple times in different lineages.
The Evolutionary Rationale
The existence of reabsorption in estrous cycles and shedding in menstrual cycles points to different evolutionary strategies for reproduction. For most mammals, reabsorbing the uterine lining if pregnancy does not occur conserves energy, avoiding the cost of rebuilding it each cycle.
Conversely, menstruation is linked to “spontaneous decidualization,” where the uterus prepares a specialized, thick lining for a potential embryo before implantation. If the embryo is not viable or does not implant, this resource-intensive lining is shed. One theory suggests this allows the mother’s body to screen for and reject defective embryos early, preventing costly, unsuccessful pregnancies.
Another hypothesis for menstruation is the “pathogen defense” theory, suggesting that shedding the uterine lining helps remove infectious agents or pathogens that may have entered the uterus. Furthermore, menstruating species often have more invasive placentas, where fetal tissue directly interacts with the mother’s blood vessels. The pre-building and shedding of the uterine lining may provide a protective barrier, allowing the mother more control over the implantation process and preventing the fetus from over-invading her tissues.