Menstruation is a biological process defined by the cyclical shedding of the thickened uterine lining, the endometrium, which is accompanied by bleeding. This occurs in females of reproductive age when a fertilized egg does not successfully implant in the uterus. The cycle is driven by the rise and fall of reproductive hormones, preparing the body for a potential pregnancy each time. While widely associated with humans, true menstruation is an extremely rare occurrence across the animal kingdom, found in only a few mammalian groups.
Defining Menstruation Versus Estrous Cycles
The distinction between menstruation and the estrous cycle lies in the fate of the uterine lining. True menstruation involves the spontaneous breakdown and expulsion of the endometrial tissue when progesterone levels drop at the end of an infertile cycle. This requires the uterus to actively dismantle the tissue built up in preparation for pregnancy. In contrast, the estrous cycle, the reproductive pattern of most non-primate mammals, involves the lining thickening, but if fertilization does not occur, the tissue is simply reabsorbed back into the body. This reabsorption is an efficient, energy-conserving process that avoids external bleeding. Menstruation is also linked to spontaneous decidualization, where the uterus pre-emptively prepares its lining for implantation without an embryonic signal. This preparatory action leads to the shedding of the endometrium if no pregnancy ensues.
The Exclusive List: Mammals That Menstruate
Menstruation is a highly isolated trait, appearing in less than two percent of the approximately 6,400 known mammalian species. The largest group exhibiting this trait is the order of primates, including humans, chimpanzees, bonobos, and most Old and New World monkeys. Their menstrual cycles are similar to humans, typically ranging from 21 to 37 days. Beyond primates, the trait evolved independently in a few distantly related lineages. Certain species of bats, such as the black mastiff bat and the fulvous fruit bat, experience true menstruation, displaying cyclical shedding that mirrors hormonal changes seen in primates. The phenomenon also appears in two small, non-primate mammals: the elephant shrew and the spiny mouse (Acomys cahirinus). The spiny mouse is the only known rodent to menstruate.
The Evolutionary Rarity of Menstruation
The scarcity of menstruation suggests it is not an advantageous reproductive strategy for most species. Shedding tissue and blood each cycle is metabolically costly, requiring the body to rebuild the endometrium from scratch. Reabsorbing the tissue, as estrous species do, is a more conservative use of energy. The leading theory posits that menstruation is a mechanical consequence of spontaneous decidualization (SD). SD is the preemptive differentiation of endometrial cells in response to maternal hormones, occurring before a fertilized egg arrives. This costly preparation is thought to have evolved as a defense against conflict between the mother and the embryo. In menstruating species, the embryo is often highly invasive, burrowing deeply into the uterine wall. By investing heavily in a pre-differentiated lining, the mother gains greater control, potentially rejecting compromised embryos. If a viable embryo does not implant, the pre-built lining is shed because it cannot be maintained or reabsorbed.
Reproductive Cycles in Non-Menstruating Mammals
The vast majority of mammals, including domestic animals like dogs, cats, cows, and horses, operate on an estrous cycle. They use the energy-efficient strategy of reabsorbing the uterine lining if pregnancy does not occur, recycling nutrients and preventing blood loss. The idea that female dogs “have periods” is a common misunderstanding rooted in visible discharge. A female dog in proestrus often has a bloody discharge, but this is not menstruation. This bleeding is spotting associated with high hormone levels causing vascular changes, and it does not represent the large-scale shedding of the thickened endometrium. In estrous species, the female is only sexually receptive during the brief window of estrus, or “heat,” which coincides with ovulation. This contrasts with menstruating species, where the female is typically receptive throughout the cycle because ovulation is concealed.