Menstruation, the periodic shedding of the uterine lining, is widely perceived as a universal feature of female mammalian biology. True menstruation is a remarkably rare event across the animal kingdom, limited to a tiny fraction of all mammalian species. The vast majority of mammals employ a different and more energetically conservative reproductive strategy. The distinction between these two reproductive cycles explains why only a select few species, including humans, experience what is commonly known as a period.
Defining the Difference: Menstrual vs. Estrous Cycles
The fundamental difference between the two main types of reproductive cycles lies in the fate of the endometrium, the tissue lining the uterus. In the menstrual cycle, the lining is prepared for a potential pregnancy by thickening and enriching itself with blood vessels and nutrients. If fertilization does not occur, the hormonal signal to maintain this prepared lining drops, causing the tissue to break down and be expelled from the body through the vagina, a process called menses.
The estrous cycle, which is found in most mammals, operates on a principle of reabsorption. When pregnancy does not occur in these animals, the body does not shed the uterine lining. Instead, the endometrium is completely reabsorbed back into the body, recycling the nutrients and tissue components. This process is sometimes referred to as “covert menstruation” because the tissue is still broken down, but without any external blood loss.
Sexual receptivity also distinguishes the cycles. Animals with an estrous cycle are typically only receptive to mating during a specific, hormonally driven phase known as estrus, or “heat,” which often coincides with ovulation. In contrast, mammals with a menstrual cycle do not have a defined period of overt sexual signaling or receptivity tied strictly to ovulation. This difference reflects evolutionary strategies for reproduction.
The Exclusive Club: Mammals That Menstruate
True, overt menstruation comprises only about 1.5 percent of all mammalian species. The most well-known group is the primates, including humans, chimpanzees, gorillas, and various species of Old World and New World monkeys. These species share a common reproductive mechanism where the uterine lining is shed when conception fails.
Outside of the primate order, the occurrence of menstruation appears in a few distinct and unrelated groups, suggesting the trait evolved independently multiple times. Certain species of bats, such as the black mastiff bat and the wild fulvous fruit bat, have been observed to menstruate. The tiny, insectivorous elephant shrew, or sengi, native to Africa, also exhibits a menstrual cycle.
The Cairo spiny mouse (Acomys cahirinus) is another remarkable exception to the general mammalian rule. The appearance of menstruation in such disparate groups—primates, bats, elephant shrews, and this specific mouse—demonstrates a striking case of convergent evolution. This means these species independently developed the same biological mechanism for managing their reproductive cycles.
The Biological Imperative: Why Most Mammals Do Not Menstruate
The prevalence of the estrous cycle in the majority of mammals suggests it is the most efficient reproductive strategy. The primary reason for the dominance of reabsorption is the significant energy savings it provides. Shedding and then rebuilding the entire uterine lining is a metabolically expensive process, requiring the body to lose and then replace valuable resources like iron, blood, and tissue protein.
Reabsorption, conversely, is a highly efficient form of nutrient recycling, allowing the female to conserve the energy and material invested in the endometrium. Scientists theorize that in most mammals, the uterine lining is thin enough that the body’s immune system can break it down and reuse its components with minimal energetic cost. This strategy is an economic choice that minimizes reproductive waste.
The evolution of menstruation is linked to the distinct way their embryos implant in the uterus. Primates and other menstruating mammals exhibit “spontaneous decidualization,” where the uterine lining becomes very thick and specialized before the embryo implants. This pre-thickened lining, necessary to accommodate the highly invasive nature of their embryos, becomes too dense for efficient reabsorption if pregnancy does not occur.
The shedding of this overly thick, specialized tissue is a biological necessity, as the body cannot break down the large volume of tissue quickly or completely enough. This spontaneous preparation of a robust lining may serve as a form of quality control, allowing the female to slough off the entire structure before investing significant resources into a doomed pregnancy. The estrous cycle, by contrast, only builds this extensive lining after the embryo is confirmed to be implanting.