While the visible shedding of the uterine lining is common in humans, it is a much rarer phenomenon across the animal kingdom. This raises the question: do other species experience a biological process similar to human menstruation?
Understanding Menstruation
Menstruation is the cyclical shedding of the uterine lining, the endometrium, which occurs when pregnancy does not happen. If implantation does not occur, the thickened lining, along with blood and mucus, is expelled from the body through visible bleeding. Hormones like estrogen and progesterone regulate this cycle. Estrogen promotes the thickening of the uterine lining, while progesterone helps maintain it, preparing for a potential pregnancy. A decline in progesterone levels, if no pregnancy occurs, triggers the breakdown and shedding of the uterine lining, initiating menstruation.
The Menstruating Few
Menstruation, characterized by the cyclical shedding of the uterine lining with visible bleeding, is observed in a limited number of mammalian species. Most menstruating species are primates, including Old World monkeys, apes (chimpanzees, gorillas, orangutans, bonobos), and certain New World monkeys. Their cycles share a similar physiological basis with humans, involving spontaneous decidualization where the uterine lining prepares for implantation even without an embryonic signal.
Menstruation also occurs in a few other distinct mammal groups. Several bat species, including the short-tailed fruit bat (Carollia perspicillata) and Pallas’s long-tongued bat (Glossophaga soricina), exhibit this process. Elephant shrews, also known as sengis, are another non-primate mammal that menstruates. The Cairo spiny mouse (Acomys cahirinus) is the only known rodent species to menstruate, displaying a cycle of around 8 to 9 days with approximately three days of bleeding.
The Estrous Majority
Most mammals, particularly non-primates, experience the estrous cycle, often referred to as being “in heat.” If pregnancy does not occur, the uterine lining is typically reabsorbed by the body, preventing visible external bleeding. During the estrous cycle, females are sexually receptive only during a specific fertile period, unlike menstruating species.
While some animals with estrous cycles, such as dogs, may exhibit vaginal bleeding, this is distinct from menstruation. It signifies high fertility or ovulation, not uterine lining shedding. Common examples include cows, cats, horses, sheep, and various rodents.
Why the Difference?
The reasons behind the evolutionary divergence of menstrual and estrous cycles are complex and still subjects of scientific investigation. One prominent theory, the “pathogen expulsion” hypothesis, suggests that menstruation evolved to cleanse the uterus of potential infections. By shedding the uterine lining, the body might remove sperm-borne pathogens, offering a protective mechanism against reproductive tract infections.
Another hypothesis, related to “energy conservation,” proposes that shedding the uterine lining is metabolically less costly than completely reabsorbing it. However, a leading theory focuses on “spontaneous decidualization” as a form of quality control for embryos.
In menstruating species, the uterine lining prepares for pregnancy through a process called decidualization, which occurs spontaneously even without an embryo present. This pre-preparation allows the mother’s body to screen and potentially reject non-viable embryos early on, conserving resources that would otherwise be invested in a compromised pregnancy. The independent evolution of menstruation in diverse groups like primates, bats, elephant shrews, and the spiny mouse suggests that this reproductive strategy offers distinct advantages, particularly in species with more invasive types of embryo implantation.