Menstruation is a monthly biological process involving the shedding of the uterine lining and bleeding. This phenomenon is a distinctive aspect of human female biology, leading many to assume it is unique to our species. However, most other mammals employ different reproductive strategies, with only a select few experiencing true menstruation. This distinction raises questions about the evolutionary reasons behind this human trait.
Understanding Human Menstruation
The human menstrual cycle is a finely tuned, hormone-regulated process that prepares the uterus for potential pregnancy each month. It begins with the follicular phase, where hormones like estrogen thicken the endometrium (uterine lining) with blood vessels. This preparation creates a nourishing environment for a fertilized egg.
After ovulation, if pregnancy does not occur, the levels of key hormones, particularly estrogen and progesterone, decrease. This decline signals the body that the thickened lining is no longer needed, leading to its breakdown and shedding. This shedding of the endometrium, including blood and tissue, is a menstrual period.
Diverse Reproductive Strategies in Animals
In contrast to human menstruation, most other mammals employ a reproductive strategy known as the estrous cycle. During this cycle, the female reproductive system also prepares the uterine lining for potential pregnancy. However, if fertilization does not occur, the thickened endometrium is reabsorbed by the body rather than being shed externally. This reabsorption is an efficient process, allowing the animal’s body to reclaim valuable nutrients and minimize blood loss.
Females in estrous cycles often have a distinct period of heightened sexual receptivity, known as “heat” or estrus, which coincides with ovulation. During this time, they are only receptive to mating, a clear signal of their fertility. Examples of animals with estrous cycles include dogs, cats, cows, and horses.
Theories Behind Human Menstruation
The evolutionary reasons for human menstruation, unlike the reabsorption seen in most mammals, are subjects of ongoing scientific discussion. One theory, the “energy conservation hypothesis,” suggests shedding a metabolically expensive uterine lining is more efficient than continuously maintaining it if pregnancy is not confirmed. This argues menstruation’s temporary resource loss might be a lower cost than constant energy drain.
Another theory, the “pathogen expulsion hypothesis,” proposes menstruation removes pathogens, particularly those introduced during mating, reducing infection risk. A more recent theory, “spontaneous decidualization,” suggests the uterine lining differentiates for a highly invasive embryo, making shedding necessary if implantation doesn’t occur. These theories highlight the complexity of reproductive evolution and human adaptations.
Animals That Also Menstruate
While menstruation is often considered a human characteristic, it is not exclusive to our species. A small number of non-human animals also exhibit true menstruation, involving the external shedding of the uterine lining. This phenomenon is most commonly observed in primates, sharing similar physiological mechanisms with humans regarding uterine preparation and shedding.
Specific examples among primates include:
Chimpanzees
Orangutans
Gorillas
Baboons
Macaques
Beyond primates, menstruation has also been documented in a few other diverse species, including:
The elephant shrew
Certain bat species, such as the black mastiff bat and the wild fulvous fruit bat
The Cairo spiny mouse
These instances demonstrate that while rare, the evolutionary path to menstruation has occurred independently in different mammalian lineages.