The term menstrual cycle is used to describe the hormonal and reproductive tissue changes that occur in adult females during their reproductive years. It does not represent an independent physiological process, since it is unlikely that there was a selection independent of the evolutionary events that led to it.
From the perspective of evolution, indefinite maintenance of the uterus in the state of alertness for eventual implantation would be highly counterproductive. Thus menstruation probably developed as a byproduct of increased cellular changes in the endometrium as a response to increased assertiveness of the blastocyst.
The menstruating species
In primates, menstruation is well-documented in the catarrhines which include humans, apes and Old World monkeys, while strepsirrhines (e.g. lemurs) do not menstruate. In Old World monkeys (with the rhesus monkey as the most prominent member) menarche or first menstruation occurs at about 4 years of age when compared with the great apes where it occurs at about 8-10 years among the animals living in nature.
The duration of the menstrual cycle also varies with species, and it is approximately 29 days long in orangutans, 30 days in gorillas and about 37 days in chimpanzees. It is important to note that menstrual bleeding in non-human primates is minimal. Furthermore, the lengths of the cycle and hormonal conditions have a substantial influence on the sexual behavior at the time when impregnation may occur.
A situation with bats is less clear than with the primates. Evidence for menstruation is present in at least four bat species – three phyllostomid bats (Glossophaga soricina, Desmonodus rotundus and Carollia perspicilata) and a molossid bat species Molossus ater. In addition, there is compelling evidence for menstruation in elephant shrew, which is a small-size mammal found exclusively in Africa.
Other mammalian species such as dogs and tree shrews also sometimes bleed, but in those species it does not represent menstruation. In dogs bleeding occurs from the vagina instead of the uterus, and not at the time of progesterone withdrawal. Conversely, in tree shrews blood is actually derived from the uterus, but occurs only in a specific state of that species called pseudopregnancy.
Therefore, current consensus is that menstruation is restricted to higher primates, the elephant shrew and some bats, albeit more studies are needed to grasp the full picture. For example, experimental studies have shown that mice (which do not exhibit menstruation under natural conditions) can menstruate in experimentally induced conditions.
Evolutionary theories of menstruation
Over the years, several dominant theories on evolution of menstruation were proposed by the academic community. One of them claims that menstruation evolved in order to protect the uterus and oviducts from sperm-borne pathogens by removing infected endometrial tissue and deploying immune cells to the uterus.
The alternative hypothesis is that the uterine endometrium is shed or resorbed in cases of failed implantation, since the cyclical regression and renewal is energetically less costly than maintaining the endometrium in the metabolically active state required for implantation. Following this hypothesis, the profuse bleeding of humans and chimpanzees can be attributed to the large size of the uterus relative to adult female body size and to the design of the small blood vessels in catarrhines.
Recent research goes in the direction that menstruation is a mechanistic consequence of spontaneous decidualization (a hormone-induced differentiation of the endometrium), which is supported by the correlation between this phenomenon and menstruation patterns across species. Furthermore, some experiments show that differentiated endometrial stromal cells are committed to apoptosis upon progesterone withdrawal.
Spontaneus decidualization evolved in higher primates, some bats and the elephant shrew (i.e. all the aforementioned species that menstruate) due to a conflict between mother and fetus via two possible selective forces: invasive embryos and high rate of impaired embryos. The process became genetically stable in menstruating lineages, thus allowing females to prepare for pregnancy without any signal from the fetus.
Further Reading
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