Menstruation is the periodic shedding of the uterine lining, called the endometrium. This process occurs when a fertilized egg does not implant in the uterus, signaling the body to prepare for a new reproductive cycle. It involves a complex interplay of hormones that regulate the thickening and eventual breakdown of the uterine tissue.
Not All Mammals Menstruate
Menstruation, as experienced by humans with visible external bleeding, is rare across the mammalian kingdom. In fact, true menstruation is observed in less than two percent of all mammal species. Reproductive cycles are universal among mammals, but their specific mechanisms differ.
Mammals That Do Menstruate
True menstruation is observed in a few mammalian groups. This includes humans and other primates, such as:
Chimpanzees
Bonobos
Gorillas
Various Old World and New World monkeys (e.g., baboons, macaques)
Their menstrual cycles vary in length; for instance, chimpanzees typically have cycles ranging from 28 to 45 days, while orangutans average 29 to 32 days. Humans generally experience a cycle lasting around 28 days, though a range of 21 to 35 days is considered normal.
Beyond primates, other distinct groups also exhibit menstruation. Several species of bats have been observed to menstruate, including:
The short-tailed fruit bat
The long-tongued bat
The black mastiff bat
The wild fulvous fruit bat
Their cycles typically range from 21 to 33 days, with bleeding often lasting only a single day in some species. The elephant shrew, also known as a sengi, is another non-primate mammal that menstruates, with an average cycle length of about 12 days. In the wild, elephant shrew menstruation may not be a strictly cyclical event but can occur at the end of the breeding season. The Cairo spiny mouse is a recently identified rodent species that undergoes menstruation, exhibiting a cycle of about 8 to 9 days with approximately three days of bleeding.
The Estrous Cycle: An Alternative
In contrast to menstruation, the estrous cycle is the predominant reproductive pattern among most mammals. During an estrous cycle, if pregnancy does not occur, the uterine lining is typically reabsorbed by the body rather than being shed externally. This reabsorption process is efficient, as it conserves energy and nutrients that would otherwise be lost.
Animals like:
Dogs
Cats
Cows
Deer
Many rodents
follow an estrous cycle. A key phase within this cycle is “estrus,” or “heat,” during which the female is sexually receptive and signals her readiness to mate. Estrous cycles can vary in frequency; some animals are polyestrous, cycling multiple times a year, while others are monoestrous, having only one cycle annually. Many species are also seasonally polyestrous, exhibiting multiple cycles only during specific breeding seasons.
The Biological Purpose of Different Cycles
The distinct reproductive strategies of menstruation and the estrous cycle reflect varied evolutionary and physiological adaptations. Menstruation, particularly the spontaneous thickening of the uterine lining (decidualization) before an embryo is even present, is a metabolically demanding process. One theory suggests that this pre-thickening and subsequent shedding may serve as a protective mechanism, allowing the female to discard a potentially defective embryo or an imperfect uterine lining.
Another perspective proposes that menstruation helps the uterus prepare and regenerate more frequently, which might be beneficial for species with spontaneous ovulation. Spontaneous ovulators, like menstruating mammals, release an egg regardless of mating, meaning the uterus needs to be consistently ready for implantation. In contrast, many estrous cycling mammals are induced ovulators, where the egg is only released after mating. For these species, reabsorbing the uterine lining is an energy-efficient approach, as the body only invests in a fully developed uterine lining once conception is more certain. Menstruating species also tend to have smaller litters, typically one to four offspring, and longer gestation periods, indicating a high biological investment in each pregnancy.