Evolution of endometrial cycles and menstruation

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Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species. Overt menstruation is found primarily in humans and close evolutionary relatives such as chimpanzees. The females of other placental mammal species have estrous cycles, in which the endometrium is reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. Menstruation is a natural cleansing for a woman. Margie Profet, states in her article that the blood flow of a menstrual cycle is different from the flow of regular blood flow. In the uterine cavity are macrophage cells that fight off the presence of viruses. In this case sperm is a vector of diseases and a woman's menstruation is to help cleanse the woman uterine by having a menstrual cycle every month. This applies to women that are sexually active and have unprotected sex, so that the pathogens will not colonize the uterus and the oviduct. "The uterus appears to be designed to increase its bleeding if it detects infection, as a result artificially curtailing infection-induced uterine bleeding may be contraindicated" since it interferes with her body's natural capacity to defend itself against pathogens (Profet, p. 355). Pathogens is an organism or a virus that cause a disease. These pathogens may originate in the vagina, in the cervix, or in the male reproductive system. She then describes an array of female defenses against sperm-transported pathogens in the vagina, cervix and uterus (Profet, p. 342-343). Of course, she notes, the aggressiveness of this defense system must be balanced with the need to make sperm welcome for reproductive purposes. To accomplish this balance, female defenses are increased somewhat during, but especially after, periods of sexual receptivity, that is, during and after exposure to sperm and the accompanying pathogens (Profet, p. 342). During and after sexual receptivity, the walls of the vagina become cornified or scale-like, 'hindering sexually transmitted pathogens from colonizing vaginal tissue' (Profet, p. 342). In the cervix, thick, acidic mucous accumulates to keep sperm and the accompanying pathogens from proceeding to the uterus. During sexual receptivity, this particular defense must be weak in order to allow sperm through to fertilize eggs, but before and especially after sexual receptivity this defense is particularly strong. Profet notes that the uterus and oviducts have similarly well timed defenses (Profet, p. 343-344), and argues further that 'nonmenstrual forms of normal uterine bleeding', such as postpartum and periovulatory bleeding, also have an anti-pathogen function

Only humans and bonobos (pygmy chimpanzees) have menstrual cycles (they get their periods because the endometrium is excreted). Other mammals have an estrous cycle (the endometrium is reabsorbed into their bodies). Giving birth is much more painful for humans than for other primates because of our walking upright. As we evolved to walk upright, our birth canal got twisted, so the baby starts off facing one direction but ends up the other way as it comes out. Most primates can give birth and simply pull the baby up to their breasts directly, but in humans the baby comes out facing the other way, so its spine might be damaged if we did that. That's why humans generally help each other give birth. Strassmann argues that the primary purpose of menstruation is the regrowth of the endometrium of which menstrual blood is only a side-effect. Why, she asks, do women periodically regenerate the endometrium? She finds her answer in evolutionary biology. The cyclical reconstitution of the endometrium is more cost-efficient than maintaining the health and vigor of a single entity. She argues, "edometrial economy" preserves the metabolic equivalent of six days worth of food for women an important evolutionary survival advantage for those times in human history when the food supply has been scarce, and where six days can mean the difference between life and death (Strassmann, p.181).

In Profet opinion, the composition of blood is the key. By using blood, the bacteria and necrotic tissue can be phagocytized by the leukocytes, thus reducing the risk of infection. Profet's hypothesis makes three predictions which were tested by Strassman (1996). These are the following: (1) Uterine pathogens should be more prevalent before menses than after. (2) In the life histories of females, the timing of menstruation should track pathogen burden. (3) In primates, the copiousness of menstruation should increase with the promiscuity of the breeding system. The first prediction was tested using data on women subjects and was not upheld. The second prediction was countered by the fact that in humans, sexual activity may be present, but menstruation absent during pregnancy, amenorrhea (a medical condition in which there is a lack of menstruation), and after menopause.The third predicition was tested by using data on menstrual flow and sexual activity in primates, including those general in which menstruation was absent. She used two different analyses including one which accounted for the influence of phylogeny. In the second case, menstruation had to have been gained and lost several times throughout evolutionary history.In addition, the amount of blood loss in women is positively correlated with uterine size which is asociated with height and childbirth history which Profet did not take into account.

In summary, none of Profet's predictions were upheld. Instead, Strassman proposes that the uterine endometrium is shed, or resorbed in the case of non-menstruating mammals, because it is energetically less costly than maintaining the endometrium on a continual basis. By examining the energetic costs over different phases of the menstrual cycle and assuming a daily energy requirement of 9.2 MJ/day, she estimates that over four months, nearly six days of food is saved by menstruating. Over human evolutionary history, these savings would have been advantageous when food availability was low. Strassman therefore also believes that menstruation is adaptive in function, though for a different reason.

Work Cited

  • Profet, Margie (1993) "Menstruation as a Defense Against Pathogens Transported By Sperm", Quarterly Review of Biology 68, 3(September): 335-381.
  • Strassmann, Beverly I. (1996) "The Evolution of Endometrial Cycles and Menstruation", Quarterly Review of Biology 71, 2(June): 181-221.