Menstruation is a biological process involving the regular discharge of blood and tissue from the inner lining of the uterus, or endometrium, when pregnancy does not take place. It is a cyclical event primarily driven by hormonal fluctuations within the female reproductive system. While familiar to humans, menstruation is surprisingly uncommon across the animal kingdom. It is not a universal characteristic among all mammals.
Animals That Menstruate
Menstruation, characterized by the overt shedding of the uterine lining, is primarily observed in a select group of mammals. The most well-known menstruating animals are primates, including humans, apes, and Old World monkeys. Cycle lengths vary among species; for instance, chimpanzees have an average cycle of about 36 days, while macaques typically have cycles around 30 days.
Beyond primates, menstruation is rare, found in only a few other mammalian species. Certain bat species, such as Seba’s short-tailed bat, Pallas’s long-tongued bat, and the wild fulvous fruit bat, have been observed to menstruate. Their cycles can range from 21 to 33 days, with bleeding periods lasting around 5 days. Another group includes elephant shrews, in which menstruation tends to occur at the end of their breeding season. The Cairo spiny mouse is the only known rodent to exhibit a true menstrual cycle, typically lasting 8 to 9 days with about 3 days of visible bleeding.
Reproductive Cycles in Other Mammals
The majority of female mammals do not menstruate; instead, they experience an estrous cycle, a reproductive pattern distinct from menstruation. In species with an estrous cycle, the uterine lining, or endometrium, is reabsorbed by the body if pregnancy does not occur, rather than being shed. This reabsorption minimizes tissue loss and nutrient expenditure, making it an efficient reproductive strategy for most mammals.
The estrous cycle prepares the female for reproduction and is characterized by periods of heightened sexual receptivity, often referred to as being “in heat” or estrus. During estrus, females are receptive to mating, and outward signs such as behavioral changes or physical displays may signal their fertility. These cycles are widely variable in duration and frequency depending on the species, with some animals experiencing multiple cycles per year (polyoestrous) and others having a single annual cycle (monoestrous).
Hormonal changes regulate the estrous cycle, preparing the uterus for potential implantation. If conception does not happen, the uterine lining regresses and is reabsorbed. This contrasts sharply with menstruation, where the endometrium breaks down, resulting in visible bleeding. The estrous cycle is the biological norm for most placental mammals, encompassing species from cows and sheep to dogs and horses.
Why Menstruation Occurs
The evolution of menstruation in a limited number of species suggests specific biological advantages. One prominent theory, the “energy conservation hypothesis,” proposes that shedding the uterine lining is energetically less costly than continuously maintaining a receptive endometrium. If pregnancy does not occur, dismantling and rebuilding the uterine lining might conserve more energy than sustaining it indefinitely.
Another leading explanation is the “maternal-fetal conflict hypothesis,” which suggests that menstruation is a byproduct of a highly invasive implantation process. In menstruating species, the embryo invades deeply into the uterine lining, forming an intimate connection with maternal blood vessels early in development. This invasive implantation requires the mother’s uterus to be highly selective, potentially shedding a low-quality or non-viable embryo before significant maternal resources are invested.
The process of “spontaneous decidualization,” where the uterine lining prepares for pregnancy even without an embryonic signal, is closely linked to menstruation. This preparatory state allows the mother to exert more control over embryo quality. If an embryo is not optimal, the spontaneous decidualization followed by progesterone withdrawal can lead to the shedding of the uterine lining, effectively “rejecting” the embryo. This mechanism may be particularly advantageous for species with a higher rate of genetically impaired embryos, such as humans.