Key Terms in Evolutionary Biology of Sex, Gender, and Sexuality: Menstruation

= Menstruation   =

Definition
Menstruation is, on average, a 28 day cycle experienced by females. Menstruation refers to "the periodic discharge from the uterus in the absence of pregnancy." (Dafarty and Patki, 2009) At the beginning of ovulation, one phase of this cycle, an egg is released by the ovary. An array of hormones then causes the endometrium, or the mucous membrane lining the uterus, to grow. If fertilization of the egg occurs, the early-stage embryo, now a zygote, implants itself deep inside the tissue of this tissue, thus beginning the onset of pregnancy. If pregnancy does not occur, then about two weeks after ovulation, all of the layers of the endometrium save the basal layer is shed with blood. (Dafarty and Patki, 2009) It is commonly agreed upon that menstruation is necessary for preparing the uterine lining for implantation of the embryo. (Dafarty and Patki, 2009)

Term In Context
A use of the term in context world be to explain the human menstrual cycle.

Day 1: This is the first day of vaginal bleeding. This lasts about four days. During this period, the uterine lining bleeds and loses a little bit of tissue, “due to the uterine lining being sloughed off.” (Barash and Lipton, 2009)

Day 5: Beginning of what is called the proliferative phase. In this phase, follicles in the ovary start to multiply under the influence of hormones (i.e. the follicle stimulating hormone, referred to as the luteinizing hormone). While this is happening, the uterine lining begins to thicken, largely stimulated by estrogen. One or two follicles become dominant. In response to the same luteinizing hormone, an egg is released.

Day 14: On average, this is when ovulation occurs; this is the midpoint of the cycle.

Day 15- Day 28: The developing egg has been nourished and surrounded by cells. This developing egg becomes “condensed into the corpus luteum under the influence of pituitary hormones.” The corpus luteum is also referred to as the “yellow body.” The corpus luteum secretes estrogens and progesterone, which then act on the uterine lining to encourage its growth and reception of an embryo, should one arrive. If an embryo is nowhere to be found, the yellow body begins to regress. When this occurs, progesterone levels drop and the uterine lining sheds. (Barash and Lipton 2009)

This causes the cycle to repeat, a process commonly referred to as "getting a period."

Theories
The average age of menarche, or the onset of puberty and the process of menstruation in females, is 12.5 years of age. (Dafarty and Patki, 2009) The question most evolutionary biologists ask is: why do women menstruate? Is there an adaptive purpose for menstruation? Has it been selected for? There are three theories that attempt to explain this biological phenomenon.

The Signaling Hypothesis: The theory that claims a woman is not losing blood during her period, but rather that her period is a signal. The female is actually losing blood but it is not seen as a loss becuase there is a purpose for it. “Perhaps monthly bleeding is a sociosexual signal informing other women that the signaler is entering the realm of sexual maturity.” (Barash and Lipton 2009) In reference to the signaling hypothesis, it is also possible that menses is not a signal for everyone, but only for those intimately involved with the signaler, such as her mother or grandmother. It is also possible that the signal is directed towards men, indicating a negative response towards men trying to copulate. (Barash and Lipton 2009)

The Cleansing Hypothesis: This is a theory developed by Margaret Profet in the early 1990s, stating that menstruation is a way of clearing out pathogens that a woman may receive during sexual intercourse, helping the woman to “start fresh” every month. “It is the male sexual products, sperm and semen, as well as the physical act of sexual intercourse that is likely to be unclean. The uterus, by peeling away much of its surface, regularly displaces troublemakers.” (Barash and Lipton 2009)

The Metabolic Efficiency Hypothesis: This theory proposed by Joan Strassman, an evolutionary biologist, claims it energetically less costly to grow a new uterine lining each month than to provide the energy to fix the old one. It's possible that menstruation is “an adaptive conservation of metabolic energy.” (Barash and Lipton 2009)

Image


The journey of the egg during menstruation

(www.healthofchildren.com)

History of the Use of the Term and its Origin
Ancient philosophers thought menstrual blood was the source of a new life. Aristotle theorized that blood from the uterine lining contains a substance called “material prima” (Travis 1997) that a man’s sperm shapes into an embryo during sexual intercourse. Hippocrates argued that women menstruate to remove impurities as in the later-developed signaling hypothesis, while men cleanse their bodies through the process of sweating. Parts of the Bible claim that when women have their period they are polluted and dangerous to men, while other theories claim that women generate more blood than they can physically handle, and menstruation allows them to get rid of the unnecessary blood. (Travis 1997)

Controversies, Misuses, Common Misunderstandings
A main controversy related to the concept of menstruation is the widely known idea that when women live together, for some reason, their menstrual cycles seem to synchronize; but why? It is said that this could be do to pheromones. In evolutionary theory, this synchrony could help make male infidelity less likely. If most women nearby are menstruating at the same time, men would presumably be inclined to stay home to inseminate their own mates when able as opposed to inseminating other mates. It is much wiser for the man to stay home and protect his own mate from other males during a period of synchrony than to not. However, an argument for asynchrony might be that it would actually be advantageous for a mother to avoid giving birth at the same time as females around them if there is serious competition to provide food and resources to their offspring. Biologists are not even 100% sure if the theory of synchrony is true, but studies show strong evidence for it.

Another controversy is over the question of why females bleed in larger amounts than other mammals who menstruate. Bleeding is theorized to be linked to sexual acts; therefore animals that tend to copulate more should presumably bleed more. Biologists also wonder why mammals are the only ones to menstruate while birds, lizards, and insects all experience internal fertilization as well.

Questions
1. Are there any examples of societies in which menstruation is considered a negative process? During some of my research, I read about certain Papuan tribal societies that shunned the women of the tribe when they were menstruating because of their fear of blood; did you come across any similar examples?

Author
Rebecca Waintrup