While menstruation is a familiar biological process for humans, it is a relatively rare occurrence across the vast diversity of the animal kingdom. Most mammals employ different reproductive strategies, highlighting the wide range of evolutionary paths life has taken to ensure successful reproduction.
Understanding Menstruation
Menstruation is a cyclical biological process characterized by the shedding of the uterine lining, known as the endometrium. This occurs if a fertilized egg does not implant in the uterus. The purpose of building up this thick, blood-rich lining each cycle is to create a nurturing environment for a potential embryo. If pregnancy does not happen, hormone levels, specifically progesterone, drop, signaling the body to break down and expel the uterine tissue and blood through the vagina. This discharge is distinct from other types of bleeding, such as spotting during an estrous cycle, as it involves the complete breakdown and expulsion of the uterine wall.
Animals That Menstruate
Menstruation is uncommon among mammals. Only a small fraction, fewer than 2% of all mammalian species, exhibit this process. The primary group known to menstruate includes most primates, such as humans, chimpanzees, baboons, and macaques.
Beyond primates, menstruation has been observed in a select few other species. These include certain species of bats, such as the Seba’s short-tailed bat and Pallas’s long-tongued bat, and the elephant shrew. Recently, the Cairo spiny mouse has also been identified as a menstruating rodent. The presence of menstruation in these distantly related groups suggests this reproductive strategy has evolved independently multiple times rather than being inherited from a common ancestor.
The Estrous Cycle
The vast majority of female mammals experience an estrous cycle. In stark contrast to the small number of menstruating species, this cycle also prepares the uterus for potential pregnancy, but with a key difference: if fertilization does not occur, the uterine lining is reabsorbed by the body rather than being shed externally. The estrous cycle culminates in a period of heightened sexual receptivity known as “estrus” or “heat,” during which the female is fertile and willing to mate.
The duration and frequency of estrous cycles vary widely among species. Some animals are polyestrous, meaning they can have multiple cycles throughout the year if not pregnant, like cows, pigs, and rats. Others are seasonally polyestrous, experiencing multiple cycles only during specific breeding seasons, such as horses, sheep, and goats. There are also monoestrous animals, like dogs, foxes, and bears, which typically have only one estrous cycle per year. While some animals with estrous cycles, such as dogs, may exhibit some bloody vaginal discharge, this bleeding has a different physiological origin than true menstruation and is associated with increased estrogen causing blood vessel dilation, not uterine lining shedding.
Why Reproductive Cycles Differ
The evolutionary reasons behind the differences in reproductive cycles are a subject of ongoing scientific research and debate. One prominent hypothesis suggests that menstruation, with its shedding of the uterine lining, might be a byproduct of “spontaneous decidualization.” This process involves the uterus preemptively preparing a thick, specialized lining for an embryo, regardless of whether conception has occurred. This ready-made lining could allow for more selective embryo implantation and potentially protect the parent from an overly invasive embryo by enabling the rejection of genetically abnormal or poor-quality embryos.
Another theory, the “energy efficiency hypothesis,” proposes that reabsorbing the uterine lining, as seen in the estrous cycle, is more energetically efficient for the animal than shedding and rebuilding it each cycle. However, some arguments suggest that menstruation might actually be an energy-saving mechanism in the long run by avoiding the continuous maintenance of a metabolically active endometrium if pregnancy doesn’t occur. A different perspective, the “pathogen expulsion hypothesis,” suggests that shedding the uterine lining helps to remove pathogens that might be introduced during mating, thus acting as a protective mechanism against infections. These various theories highlight the complex interplay of physiological costs, benefits, and evolutionary pressures that have shaped the diverse reproductive strategies observed in the animal kingdom.