Menstruation is the cyclical shedding of the uterine lining, or endometrium, when conception does not occur. This biological process is regulated by hormonal fluctuations within the female reproductive system. While common for humans, menstruation is a unique reproductive strategy that is rare among mammals. This distinct mechanism sets a few select mammalian groups apart.
Mammals That Menstruate
True menstruation, the spontaneous shedding of the uterine lining, is observed in a limited number of mammalian species. Primates, including humans, apes, and Old World and New World monkeys, are the most recognized group that menstruates. Chimpanzees, bonobos, and baboons, for example, exhibit menstrual cycles similar to humans, though the duration and intensity of bleeding can vary between species.
Beyond primates, menstruation is also found in a few other distinct mammalian lineages. Certain bat species, such as Seba’s short-tailed bat and Pallas’s long-tongued bat, display regular menstrual cycles. The elephant shrew, a small insectivorous mammal, is another unique example. The Cairo spiny mouse has also been identified as having a true menstrual cycle, making it the only known rodent to do so. In these menstruating species, the uterus undergoes spontaneous decidualization, where endometrial cells prepare for embryo implantation even without a fertilized egg. If pregnancy does not occur, this prepared lining is then shed.
Mammals With Estrous Cycles
The majority of mammals undergo an estrous cycle. In this cycle, the uterine lining is reabsorbed by the body if pregnancy does not occur, rather than being shed externally. This reabsorption ensures no tissue is wasted, representing a different biological approach to reproductive readiness.
During the estrous cycle, females are only sexually receptive during a specific phase called “estrus,” often referred to as “heat.” This period is characterized by noticeable behavioral and physiological changes, signaling fertility. Examples of mammals with estrous cycles include dogs, cats, cows, mice, and horses. The length and frequency of estrous cycles vary widely among these species; for example, dogs typically experience heat twice a year, while rats have very short cycles.
Theories Behind Menstruation
Several scientific hypotheses explore why some mammals menstruate while most others do not. One theory, the “energy conservation hypothesis,” suggests that shedding the uterine lining is more energetically efficient than continuously maintaining a prepared uterus if pregnancy is not guaranteed. However, this idea has faced criticism, as menstruation itself involves metabolic costs.
Another prominent theory is the “pathogen defense hypothesis.” This proposes that menstruation protects the uterus from pathogens introduced during mating. The shedding of the endometrial lining helps flush out potential infectious agents and introduces immune cells, thereby reducing the risk of infection.
The “maternal-fetal conflict hypothesis” suggests menstruation is a byproduct of the mother’s control over embryo invasiveness. In menstruating species, the uterus undergoes spontaneous decidualization, preparing a highly selective environment for the embryo. This mechanism allows the mother’s body to screen for and reject genetically weaker embryos before significant resources are invested, giving her more reproductive control. These theories are not mutually exclusive, and the evolution of menstruation may involve a combination of these and other factors.