The term “period” in humans refers to menstruation, a regular biological process that occurs as part of the female reproductive cycle. It involves the monthly shedding of the uterine lining, known as the endometrium, when pregnancy does not occur. This shedding results in a discharge of blood and tissue through the vagina. This hormonally regulated process prepares the uterus for potential pregnancy.
Which Animals Menstruate
True menstruation is a relatively rare phenomenon among mammals. While most mammals undergo reproductive cycles, only a select few species experience this process in a manner similar to humans. This includes humans, great apes like chimpanzees and orangutans, and Old World monkeys such as baboons and macaques.
Beyond primates, true menstruation has been observed in a few other animal groups. These include certain species of bats, notably the Seba’s short-tailed bat and Pallas’s long-tongued bat. The elephant shrew, a small insectivorous mammal, also exhibits a menstrual cycle. Additionally, the Cairo spiny mouse is a rodent species known to menstruate.
Understanding the Estrous Cycle
In contrast to menstruation, the vast majority of female mammals undergo an estrous cycle. This reproductive cycle prepares the female for breeding, but the fate of the uterine lining differs significantly from menstruation. In an estrous cycle, if pregnancy does not occur, the thickened uterine lining is reabsorbed by the body rather than shed externally.
Visible bleeding is generally absent or minimal in animals with an estrous cycle, often limited to a small amount of spotting in some species, such as dogs. This minor discharge is not equivalent to the substantial shedding seen in menstruation. Common examples of animals that undergo an estrous cycle include dogs, cats, cows, horses, sheep, and deer.
Distinguishing Menstruation from Estrous Cycles
The fundamental difference between menstruation and the estrous cycle lies in the fate of the uterine lining if pregnancy does not occur. In a menstrual cycle, the uterine lining is shed and expelled from the body, resulting in visible bleeding. This process prepares the uterus for a new cycle of thickening.
Conversely, during an estrous cycle, the uterine lining is reabsorbed by the animal’s body if conception does not happen. This reabsorption prevents the external bleeding characteristic of menstruation. While both cycles are hormonally regulated processes aimed at facilitating reproduction, their mechanisms for managing the uterine tissue differ considerably. The presence or absence of significant external bleeding is a primary indicator distinguishing these two types of reproductive cycles. Furthermore, in species with estrous cycles, females are typically only receptive to mating during a specific “heat” period, whereas menstruating species may be sexually receptive throughout their cycle.
Why Menstruation Evolved
The evolution of menstruation in the few species that experience it remains a subject of scientific inquiry, with several theories proposed. One hypothesis suggests that shedding the uterine lining is energetically more efficient than continuously maintaining a metabolically active lining when pregnancy does not occur. This “energy conservation” theory posits that the cost of rebuilding the lining is less than the continuous upkeep.
Another theory, often referred to as the “pathogen defense” hypothesis, suggests that menstruation evolved as a way to remove potential pathogens that might be introduced into the uterus, for instance, by sperm. The shedding of the uterine tissue could help cleanse the reproductive tract. A more recent perspective highlights “spontaneous decidualization” as a primary adaptive mechanism, where the uterine lining prepares for implantation independently of an embryo, and its shedding is a consequence if implantation fails or the embryo is not viable. This process may help protect the mother from potentially invasive or defective embryos.