Introduction
Menstruation is usually thought to be unique to primates, despite minor definitional differences across sources. There are differing opinions on the evolutionary benefits of shedding blood associated with rupturing the uterine lining as opposed to absorbing it, as most animals do, and the emergence of overt menstruation in humans and other species. Differences in the ovulation process are probably the cause.
Menstruation: Is it in Humans Only?
While it is entirely absent in strepsirrhine primates and perhaps weakly present in tarsiers, it is prevalent in simians (Old World monkeys, New World monkeys, and apes). Humans and close relatives like chimpanzees are the only animals that exhibit overt menstruation. This is defined as bleeding from the uterus via the vagina.
Only 10 monkey species, four bat species, the elephant shrew, and one species of spiny mouse have a menstrual cycle, but the majority of female mammals have an estrous cycle. Given the lack of close kinship between these taxa, menstruation most likely originated as a result of four separate evolutionary processes.
Menstruation, Other Species
At the conclusion of their reproductive cycle, females of other species of placental mammals go through estrous cycles, during which the animal totally reabsorbs the endometrium (covert menstruation). This is considered by many zoologists to be distinct from a “real” menstrual cycle. The physical indicators of an estrous cycle phase, which signal the animal’s readiness for insemination, are observed in female domestic animals utilized for breeding, such as dogs, pigs, cattle, or horses.
The Big Difference
Most mammalian females use pheromones, visual behavioral signals, or both to communicate fertility to males. Oestrus, “estrus,” or heat are the terms used to describe this proclaimed time of fertility. Females in estrous animals are often only open to copulation during periods of heat (dolphins are an exception). The uterus reabsorbs the endometrium in the estrous cycles of the majority of placental animals in the absence of conception. Sometimes, this endometrial disintegration without vaginal discharge is referred to as stealth menstruation.
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Humans and close evolutionary cousins like chimpanzees are the only animals that exhibit overt menstruation, which is defined as blood flowing from the vagina. Some animals, such as household dogs, have mild vaginal bleeding when it gets hot. This discharge has a physiological reason other than menstruation.
Menstruation: A Gift From Evolution
How is mensturation even a gift? How is it an advantageous outcome of evolution? Well the answer lies in thickening of the protective wall around the uterus.
The uterine lining of the majority of female placental animals thickens when ovulation commences. It then continues to grow in thickness and blood flow following the successful implantation of a fertilized egg. Decidualization is the term for this last thickening step, which is often brought on by hormones the embryo releases.
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In humans, hormonal cues from the ovaries cause decidualization to occur spontaneously at the start of each menstrual cycle. Thus, regardless of whether an egg is fertilized or implants successfully in the uterus, the human uterine lining thickens completely during each cycle. This is a defense against trophoblast invasion of the endometrial wall. By thickening the protective layer around the endometrial wall, menstruating animals essentially regard every cycle as a potential pregnancy. However, non-menstruating placental mammals do not initiate the pregnancy process. They wait until a fertilized egg gets implanted in the uterine wall.
Menstruation: The Advantage
Menstruation is thought to be a consequence of spontaneous decidualization, which developed in some placental animals because it was superior to non-spontaneous decidualization. Because spontaneous decidualization increases selectivity over the implanted embryo, it gives the mother more control over the maternal-fetal conflict. Given the disproportionately high prevalence of genetic illnesses in humans and other primates, this may be required.
Explanation
Evidence suggests that endometrial stromal cells in the uterus may identify some defects in the developing embryo. However, this is only after they differentiate into decidual cells. This results in epigenetic modifications that hinder placenta development, preventing the embryo from implanting and allowing it to be extracted during the subsequent menstrual cycle. In species whose decidualization is regulated by hormone triggers from the embryo, this failsafe mechanism is not feasible. This idea, commonly known as the “choosy uterus theory,” postulates that in species with high rates of aneuploidy and, consequently, a high percentage of “doomed” embryos, the benefits of menstruation exceed the drawbacks.