Menstruation is a biological process where the inner lining of the uterus, known as the endometrium, thickens in preparation for a potential pregnancy and is then shed if fertilization does not occur. This cyclical shedding involves the discharge of blood and tissue through the vagina. While menstruation is a widely recognized aspect of human biology, its occurrence in other animal species presents a fascinating area of scientific exploration.
Which Animals Experience Menstruation
Menstruation is a relatively rare phenomenon across the animal kingdom. It is most commonly observed in certain primate species, including great apes like chimpanzees, and Old World monkeys such as baboons, macaques, and mangabeys. Their menstrual cycle lengths can vary from 14 to 35 days. Not all primates menstruate; lemurs, for example, do not, and tarsiers exhibit only a weak form.
Beyond primates, a few other mammalian groups also experience menstruation. Several bat species are known to menstruate, although this is not universal across all 1,400 bat species. Examples include Seba’s short-tailed bat, Pallas’s long-tongued bat, the Black mastiff bat, and the wild fulvous fruit bat (Rousettus leschenaulti), with cycle lengths ranging from 21 to 33 days. The elephant shrew, also called a sengi, is a small African insectivore that also menstruates. Their menstruation can occur at the end of a breeding season if pregnancy does not happen, with an average cycle of 12 days. The Cairo spiny mouse (Acomys cahirinus) is the only rodent species known to menstruate, exhibiting a cycle of about 8-9 days with roughly three days of bleeding.
Diverse Reproductive Cycles in the Animal Kingdom
Most mammals do not menstruate; instead, they experience an estrous cycle, a different reproductive strategy. In an estrous cycle, the uterine lining thickens in preparation for pregnancy, but if conception does not occur, the lining is typically reabsorbed by the body rather than shed externally. This reabsorption process prevents any significant external bleeding. Females undergoing an estrous cycle are generally only receptive to mating during a specific phase known as “estrus” or “heat,” which is often accompanied by distinct behavioral and physiological changes.
The estrous cycle consists of several phases: proestrus, estrus, metestrus or diestrus, and anestrus. During proestrus, follicles in the ovary begin to grow, followed by estrus, when the female becomes sexually receptive and ovulation typically occurs. Metestrus and diestrus involve the activity of the corpus luteum, which produces progesterone, while anestrus is a resting phase of sexual inactivity. Many common mammals, such as dogs, cats, cows, sheep, deer, horses, pigs, and rodents like mice and rats, operate on an estrous cycle. The frequency and duration of these cycles vary widely among species, with some being polyestrous (cycling multiple times a year) and others being monoestrous (cycling once a year) or seasonally polyestrous.
The Evolutionary Insights into Menstruation
The presence of menstruation in a limited and diverse set of mammals suggests it has evolved independently multiple times rather than being inherited from a single common ancestor. This rarity has led scientists to propose several hypotheses regarding its evolutionary advantages. One theory, the “energy conservation hypothesis,” suggests that shedding and rebuilding the uterine lining might be less energetically demanding than continuously maintaining a metabolically active, thick endometrium in the absence of a pregnancy. However, this idea is debated, as shedding and regenerating tissue could also be energetically costly.
Another prominent explanation is the “maternal-fetal conflict hypothesis,” also known as the “choosy uterus theory.” This hypothesis posits that menstruation is a consequence of spontaneous decidualization, a process where the uterus prepares for an embryo that is particularly invasive, as seen in humans and some other menstruating species. By thickening the lining and then shedding it if implantation is not successful or if the embryo is not viable, the mother’s body screens and potentially rejects genetically flawed embryos, thus exerting greater control over reproductive outcomes. The “pathogen defense hypothesis” proposes that menstruation serves to protect the uterus from pathogens, such as those introduced during copulation, by flushing out infected tissue and immune cells. Despite these theories, the precise evolutionary reasons for menstruation remain a subject of ongoing scientific discussion.