The concept of menstruation is often associated with humans. However, this biological process occurs across the animal kingdom, though its presence and mechanisms in other species are less understood. Exploring this topic reveals the diverse reproductive strategies mammals employ and how these cycles differ.
Understanding Menstruation
Menstruation is a reproductive phenomenon characterized by the cyclical shedding of the uterine lining, the endometrium. This process occurs in sexually mature females when pregnancy does not take place. The shedding results in visible bleeding through the vagina. Hormonal fluctuations regulate this cycle, preparing the uterus for a potential embryo.
Mammals That Menstruate
True menstruation, involving overt bleeding and shedding of the uterine lining, is uncommon among mammals. While humans are known to menstruate, this process also occurs in a few other species. Most Old World and New World monkeys, and apes like chimpanzees, gorillas, and baboons, experience menstrual cycles similar to humans. Their cycles typically range from 21 to 37 days, with bleeding lasting a few days.
Beyond primates, menstruation is observed in other mammalian lineages. Certain bat species, including the Seba’s short-tailed bat and the fulvous fruit bat, exhibit this cycle, with some having cycles lasting 21 to 33 days. Elephant shrews also menstruate, though their cycles in the wild may align with breeding seasons rather than continuous monthly events. The spiny mouse, Acomys cahirinus, is the only known rodent species to undergo true menstruation.
The Estrous Cycle: The Common Alternative
While menstruation is limited to a few mammalian groups, most female mammals undergo an estrous cycle. This cycle also prepares the reproductive system for potential pregnancy, but it differs in how the uterine lining is handled if conception does not occur. Instead of shedding the endometrium, animals with an estrous cycle typically reabsorb the uterine lining. This reabsorption minimizes external discharge and conserves biological resources.
The estrous cycle is divided into phases marked by hormonal and physiological changes. Proestrus involves ovarian follicle growth and rising estrogen, preparing the female for receptivity. Estrus, often called “heat,” is the period of peak sexual receptivity when the female is most likely to mate and ovulation occurs. Following estrus, metestrus and diestrus involve the corpus luteum’s development and function, producing progesterone to support a potential pregnancy. If pregnancy does not occur, the uterine lining is reabsorbed, and the cycle begins anew.
Evolutionary Perspectives on Reproductive Cycles
The existence of both menstrual and estrous cycles raises questions about their evolutionary advantages. One proposed theory for menstruation’s evolution is the “pathogen defense” hypothesis. Shedding the uterine lining helps expel pathogens, such as bacteria carried by sperm, preventing uterine infections. This process, aided by immune cells, cleanses the reproductive tract.
The “energy conservation” theory is also debated. Reabsorbing the uterine lining, as in the estrous cycle, might be more energy-efficient than building and shedding the endometrium. Conversely, menstruation might avoid the energy cost of constantly maintaining a thick uterine lining.
A third theory, the “choosy uterus” hypothesis, suggests menstruation allows for removing defective embryos by shedding the uterine lining if implantation is unsuccessful or the embryo is not viable. This protects the female from investing resources in a non-viable pregnancy.
The invasive nature of embryo implantation in menstruating species, where the embryo deeply embeds into the uterine wall, is also a factor. This deep connection might necessitate a more thorough shedding mechanism to prevent complications if pregnancy fails. The precise evolutionary pressures leading to menstruation versus the estrous cycle remain active areas of scientific investigation.