Most female mammals have reproductive cycles, but the difference between humans and most animals lies in what happens to the uterine lining when pregnancy does not occur. Menstruation is the periodic shedding of the uterine wall, or endometrium, through the vagina. Most mammals, however, reabsorb the uterine lining back into the body, a process sometimes called covert menstruation. Understanding this distinction requires examining the fundamental biological processes governing reproduction in humans versus other mammals.
The Human Menstrual Cycle
The human reproductive strategy involves spontaneous decidualization. This means the endometrium thickens significantly and undergoes differentiation in anticipation of a potential pregnancy, even if no fertilized egg is present. This preparatory phase is triggered by the rise of progesterone after ovulation, transforming the tissue into a highly specialized, vascularized structure known as the decidua.
This pre-emptive preparation creates a robust, metabolically rich environment designed to support the deep and invasive implantation characteristic of human pregnancy. If conception does not happen, the body must dispose of this tissue. Because the endometrium is thick and complexly built up, it cannot be fully reabsorbed efficiently. Instead, the body actively sheds the entire upper layer of the decidua, resulting in the visible blood and tissue loss that defines menstruation.
The Standard Mammalian Estrous Cycle
The reproductive cycle common to the majority of placental mammals, such as dogs, cows, and rodents, is known as the estrous cycle. Unlike the human cycle, the endometrium in these species does not undergo spontaneous thickening in anticipation of pregnancy. Instead, the uterus prepares a thinner, less invasive lining. The term “estrus” describes the brief, fertile period when the female is receptive to mating, often called “being in heat.”
The decidual reaction in these animals is typically induced, meaning the uterine lining only transforms after the early embryo implants and sends chemical signals. If the egg is not fertilized or fails to implant, the thin endometrial tissue is completely reabsorbed by the body. This reabsorption leaves little visible blood loss, which is why these cycles often appear hidden. Although some animals like dogs may exhibit minor bloody discharge, this results from ruptured capillaries during the proestrus phase, not the active shedding of the uterine lining.
The Evolutionary Reason for Shedding
The difference between shedding and reabsorbing the endometrium relates to the depth of placental invasion. Humans develop a highly invasive type of placenta, called hemochorial, where fetal tissue penetrates deep into the uterine wall and contacts maternal blood. This deep invasion requires a robust defense mechanism from the mother to protect her body from potentially destructive fetal tissue. Spontaneous decidualization provides this defense by pre-arming the uterus.
The resulting thick decidua acts as a biological filter or quality control mechanism before the embryo accesses the mother’s blood supply. This allows the mother’s body to reject poor-quality or compromised embryos early, preventing significant damage from deep implantation. The high metabolic cost of building this thick lining is balanced by the need for intense quality control in species with invasive placentation. Shedding the tissue is not the goal itself, but the mechanistic consequence of building such a specialized, disposable structure in advance.
Another hypothesis suggests that shedding the lining is an energy-saving strategy. If the spontaneously decidualized lining is not used for pregnancy, it is less costly to shed it completely and start fresh than to maintain it in a constant state of readiness. Studies indicate that the endometrium’s metabolic rate can drop nearly sevenfold when it is in a regressed state following shedding. This cyclical renewal provides an energetic benefit, making active shedding the most efficient way to reset the system when reproduction is not guaranteed.
Animals That Also Menstruate
The premise that no animals menstruate besides humans is incorrect, though the trait is extremely rare among mammalian species. Overt menstruation—the spontaneous decidualization and subsequent visible shedding of the endometrium—is shared by a few specific groups. The most well-known examples are the higher primates, including chimpanzees, gorillas, and many species of Old World and New World monkeys.
Beyond the primate order, true menstruation has also been definitively documented in a handful of non-primate mammals:
- The elephant shrew, a small insectivore native to Africa.
- Several species of bats, such as the black mastiff bat.
- The Cairo spiny mouse, a rodent, which exhibits spontaneous decidualization and overt menstrual bleeding.
In all these cases, the presence of menstruation is linked to a reproductive strategy that, like humans, involves spontaneous uterine preparation and a more invasive form of placentation.