Menstruation is a biological process involving the cyclical shedding of the uterine lining, known as the endometrium. This shedding occurs when pregnancy does not happen, preparing the uterus for a new reproductive cycle. While commonly associated with humans, this physiological event is surprisingly rare across the broader animal kingdom.
Animals That Experience Menstruation
Most primates, including humans, great apes like chimpanzees and gorillas, and Old World monkeys such as baboons and macaques, experience menstruation. This makes primates the most widely recognized group to share this reproductive characteristic. However, the amount of bleeding can vary significantly, often being minimal and less noticeable in non-human primates compared to humans.
Beyond primates, a select few other mammalian species also menstruate. Certain bat species, including the short-tailed fruit bat (Carollia perspicillata) and the black mastiff bat (Molossus ater), have menstrual cycles. These cycles range from 21 to 33 days, with bleeding periods lasting around five days.
The elephant shrew, particularly the Eastern rock elephant shrew (Elephantulus myurus), is another group that menstruates. These small African mammals exhibit a menstrual cycle similar to humans, though in wild populations, it often occurs at the end of their breeding season. The Cairo spiny mouse (Acomys cahirinus) is the only known rodent species to spontaneously menstruate, providing a unique model for studying this phenomenon outside of primates.
The Biological Reasons for Menstruation
Menstruation in these species is linked to spontaneous decidualization. This involves the uterine lining, or endometrium, preparing for potential pregnancy by thickening and undergoing cellular changes, even without embryo implantation. This pre-preparation distinguishes it from most other mammals. If fertilization and implantation do not occur, this prepared lining is then shed.
Several hypotheses explain why menstruation evolved as a reproductive strategy. One theory suggests it is an energy-saving mechanism; shedding a metabolically active, pre-prepared uterine lining is less energetically costly than maintaining or reabsorbing it if pregnancy fails. Another idea proposes menstruation serves as a protective measure, expelling potentially defective embryos or pathogens from the uterus, acting as quality control.
The shedding process also plays a role in managing the maternal immune response. The uterus is a unique environment where the mother’s immune system must tolerate the embryo, a semi-foreign entity. Menstruation helps prevent an overly aggressive immune reaction against the developing embryo, ensuring a more hospitable environment for implantation. These mechanisms and their evolutionary advantages underscore this distinctive reproductive process.
Reproductive Strategies Without Menstruation
The vast majority of mammalian species, unlike those that menstruate, utilize an estrous cycle for reproduction. This cycle is characterized by distinct phases where the female is only receptive to mating during a specific fertile window, often referred to as “heat” or estrus. During this period, hormonal changes signal fertility and encourage mating behavior.
In species with an estrous cycle, the uterine lining also thickens for pregnancy. If conception does not occur, this lining is reabsorbed by the body rather than being shed externally. This reabsorption distinguishes it from menstruation, conserving bodily resources.
In many non-menstruating mammals, significant changes to the uterine lining, known as decidualization, occur only after an embryo has implanted. This “induced decidualization” is triggered by the embryo’s presence, contrasting with the spontaneous preparation in menstruating species. While some animals like dogs may exhibit minor vaginal bleeding during their estrous cycle, this bleeding originates from different physiological processes and is not considered true menstruation.