Many people assume that the reproductive cycle involving visible blood loss is a universal feature of female mammals, but this perception is a generalization drawn primarily from human experience. True menstruation, defined by a specific biological process, is a remarkably rare event across the vast diversity of the mammalian class. Understanding the fundamental difference in how various species prepare the uterus for potential pregnancy requires moving beyond surface-level observations of bleeding.
Menstruation vs. Estrus: Defining Reproductive Cycles
The reproductive lives of female placental mammals are governed by one of two primary cycles: the estrous cycle or the menstrual cycle. The fundamental distinction between these two lies in the fate of the endometrium, which is the specialized lining of the uterus built up to receive a fertilized egg. In the menstrual cycle, the endometrium is spontaneously shed from the body if pregnancy does not occur, a process that includes the rupture of blood vessels and results in visible external bleeding.
The estrous cycle, often referred to as “being in heat,” manages the uterine lining differently. If fertilization fails, the animal’s body reabsorbs the endometrium instead of shedding the tissue. This biological process is distinct from menstruation because the breakdown and renewal occur without external discharge.
The defining factor separating the two cycles is the biological mechanism of disposal: shedding versus reabsorption. Animals in an estrous cycle are receptive to mating only during the estrus phase, which coincides with ovulation. In contrast, animals with a menstrual cycle are often receptive throughout their cycle, irrespective of the fertile window.
The Exclusive Club: Mammals That Menstruate
The number of species that experience true menstruation is surprisingly small, representing only a tiny fraction of the approximately 6,400 known mammal species. This exclusive group is scattered across the mammalian family tree, suggesting the trait evolved independently multiple times. The majority of menstruating mammals are primates, which include all Old World monkeys, apes, and humans, such as chimpanzees, gorillas, and rhesus macaques.
Beyond the primates, the trait appears in a few isolated and evolutionarily distant mammal groups. Certain species of bats are known to menstruate, including the wild fulvous fruit bat and the black mastiff bat. The cycle observed in the wild fulvous fruit bat, lasting about 33 days, shows a hormonal pattern similar to that observed in humans.
One of the most ancient and phylogenetically distant groups to exhibit this cycle is the elephant shrew, or sengi, a small African mammal. Finally, a single rodent species, the Cairo spiny mouse, was recently discovered to exhibit spontaneous, true menstruation.
How Most Mammals Manage the Reproductive Cycle
The vast majority of placental mammals, including domesticated and wild species, utilize the estrous cycle to manage their reproductive readiness. This strategy relies on the body efficiently dismantling and reabsorbing the endometrial lining if a pregnancy does not occur.
Animals that follow this pattern include common species like dogs, cats, cows, horses, and most species of rodents and ungulates. For instance, a female dog experiences a proestrus phase often accompanied by a bloody discharge, leading to the misconception that dogs menstruate. This bleeding is generally due to capillary breakage in the vaginal lining as estrogen levels rise, not the sloughing of the uterine lining that defines true menstruation.
The reabsorption mechanism is highly efficient, conserving the valuable nutrients and iron contained within the uterine tissue. This energy-saving approach is the primary method of reproductive cycling for the mammalian kingdom.
Why Menstruation is Biologically Rare
The rarity of menstruation suggests that the estrous cycle, with its reabsorption mechanism, is the more energy-efficient and successful reproductive strategy. The energy conservation hypothesis proposes that reabsorbing the nutrient-rich endometrium is metabolically advantageous compared to shedding it externally. For most species, the cost of building the lining is lower than the cost of losing and replacing those resources.
A more complex theory, the spontaneous decidualization hypothesis, suggests that menstruation is a consequence of an aggressive type of placental development. Species that menstruate, including humans, tend to have highly invasive placentas, where the embryonic tissue penetrates deep into the uterine wall. Menstruation may be an evolutionary strategy to spontaneously prepare the uterus for a deep-diving embryo or to quickly discard a potentially defective lining before the highly invasive embryo can implant. By shedding the lining, the female prevents an inadequate or non-viable embryo from establishing a connection that would be costly to terminate later.