Möbius Strip Tease No. 42

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MENSTRUATION

The Mobius Strip Tease, Issue 42

1. TABLE OF CONTENTS

1. Table of contents
   
2. Background on the biology of menstruation
   
   1. The endometrium and menstruation
      
   2. Prevalence of menstruation among mammals
      
   3. Endometrium shedding and menstruation in humans
      
3. Why humans menstruate
   
   1. Menstruation may not be a significant energy cost
   2. Menstruation evolved in a primate ancestor
      
   3. Menstruation may fight pathogens but this is a silly theory
      
   4. Menstruation may be a byproduct of providing more nutrients to the fetus
      
4. A couple notes on the social role of menstruation
   
5. References
   
6. Appendix I (Leviticus 15:19-33)

2. BACKGROUND ON THE BIOLOGY OF MENSTRUATION

a. The endometrium and menstruation

The endometrium is a mucosa tissue full of lots of blood which lines the uterus. The fertilized egg implants itself in the endometrium, and then this tissue provides the interface by which the mother supplies nutrient to the growing embryo.

Menstruation occurs when the endometrium is shed and then expelled in menstrual fluid. Overt menstruation is when enough menstrual fluid expelled to be highly visible (as is the case in humans).

b. Prevalence of menstruation among mammals

Overt menstruation is recorded in 48 species of primate, most of which are old world monkeys and apes (the group which includes humans). Other primates that overtly menstruate include a few species of new world monkeys and the Malayan flying lemur (Cynocephalus varregatus). Overt menstruation is also occurs in shrews, being reported in 10 species of insectivore from the genera Elephantulus, Erihaceus, Tana, and Tupaia. Other species for which overt menstruation has been reported include four species of bat (Carollia perspicillata, Desmodus rotundus, Glossophaga soricina, and Molossus ater), a marsupial cat (Dasyurus viverrinus), the coyote (Canis latrans), and the African elephant (Loxodonta africana). The reports for menstruation in the coyote and African elephant are anecdotal.

c. Endometrium shedding and menstruation in humans

In humans, if the egg is not fertilized the endometrium is shed about two weeks after ovulation. The endometrium is then built back up to prepare for the next ovulation and possible fertilization that will occur two weeks later. It may seem like a waste of effort to be constantly shedding and rebuilding the endometrium, but it is actually energy effective. When a human female has a fully formed endometrium her energy requirements are between 7 and 10 percent higher than they are immediately after menses. It is simply too metabolically costly to maintain the endometrium in a state of constant readiness, especially when it is only useful once every 28 days.

Most mammals shed their endometrium if fertilization is unsuccessful, and then rebuild it in readiness for the next ovulation. It is the efficient way to do things for mammals that are only fertile on a cyclic basis. What makes overt menstruators such as humans different from the majority of other mammals is that we do not reabsorb all of our endometrium when it is shed. Human females only reabsorb about 67% of the shed endometrium, and the rest is expelled as menstrual fluid. The question then is, "Why don't human females reabsorb all of their endometrium when it is shed?". Humans do reabsorb most of the endometrium, but it would seem more efficient if they reabsorbed all of it.

3. WHY HUMANS MENSTRUATE

a. Menstruation may not be a significant energy cost

The energy and nutrient lost by menstruation may not be very significant. The human female loses an average of 40 mL of blood per menstruation. Over the period of an entire menstrual cycle, the protein lost from menstruation amounts to between 0.01% and 0.06 % of the required intake. This is not very much, and far less than the amount of protein that is lost in fecal matter. It also should be considered that menstruation would be a relatively rare event if no contraceptives were used, which was probably the case through much of our evolutionary history. Reproductive females in pre-industrial societies may only menstruate an average of once a year.

b. Menstruation evolved in a primate ancestor

There seems to be a significant phylogenetic component causing overt menstruation in humans. The majority of species that overtly menstruate are our closest relatives. It is probably the case that the trait of overt menstruation evolved in ancestral Catarrhini (old world monkeys, apes, humans) at least twice, and was subsequently inherited by a large percentage of the species in this group. But why would the Catarrhini have evolved to menstruate?

c. Menstruation may fight pathogens but this is a silly theory

Sperm are nasty little vectors that transmit pathogens from the male and female genitalia into the female's uterus. It has been hypothesized that menstruation is a way of removing these pathogens. There are several problems with this theory. If menstruation did remove pathogens, one would expect the uterine pathogen load to be lower after menses than it was before menses. Although there isn't much research on this, the research that has been done indicates that this is not true (Bollinger 1974; Hemsell et al. 1989). There also doesn't seem to be any evidence that menstrual blood reduces cervical or vaginal pathogens. Another problem with this theory is that it seems counterintuitive that blood can be a very good antibiotic. Blood contains iron, amino acids, proteins, and sugars, all of which are good for bacteria. There is evidence that blood in the reproductive tract can actually increase pathogen loads, and seems to be associated with Toxic Shock Syndrome. It is also the case that no other mammalian tissue bleeds to fight infection, and no other animals besides mammals menstruate even though most of them have dirty little sperm swimming up their reproductive tracts.

d. Menstruation may be a byproduct of providing more nutrients to the fetus

The primates that overtly menstruate tend to have a relatively high litter mass to body mass ratio. Because of this the uterus is larger in relation to the size of the mother, and it has more blood in it. The menstruating primates also tend to produce a thicker endometrium than non-menstruating primates, and they tend to have larger, spiral artioles in their endometrium instead of smaller, straight ones. These factors all contribute to there being more blood produced when the endometrium is shed. It may be that there is just too much blood for it all to be effectively reabsorbed. It is possible that some primates evolved a larger, juicier uterus in order to provide more nutrients to their offspring in the womb, and that overt menstruation is merely an incidental trait.

4. A COUPLE NOTES ON THE SOCIAL ROLE OF MENSTRUATION

One study has shown that women who are comfortable with menstruation as a normal, publicly acceptable event are also more comfortable with their personal sexuality (Rempel and Baumgartner 2003). The study also found that women who had engaged in sexual relations with their current partner during menses were significantly more comfortable with menstruation, more aroused by romantic sexual activities, and less sensitive to disgust.

However, this obviously shows a degradation of society, because according to the word of God (Leviticus 15:19-33; see Appendix I) a menstruating women is unclean and must be separated from everyone else for seven days. Anyone or anything that comes in contact with a menstruating women is defiled. Only after the woman sacrifices two doves on the eighth day as atonement for her sin can she again return to her family.

5. REFERENCES

Bollinger, C. C. 1964. Bacterial flora of the nonpregnant uterus: a
new culture technic. Obstetrics and Gynecology 23: 251-255.

Hemsell, D. L., V. L. Obregon, M. C. Heard, and B. J. Nobles. 1989.
Endometrium bacteria in asymptomatic nonpregnant women. Journal of
Reproductive Medicine 34: 872-874.

Profet, M. 1993. Menstruation as a defense against pathogens
transported by sperm. Quarterly Review of Biology 68: 335-386.

Rempel, J. K., and B. Baumgartner. 2003. The relationship between
attitudes towards menstruation and sexual attitudes, desires, and
behavior in women. Archives of Sexual Behavior 32: 155-163.

Strassmann, B. I. 1996. The evolution of endometrial cycles and
menstruation. Quarterly Review of Biology 71: 181-220.

6. APPENDIX I

Leviticus 15: 19-33

And if a woman have an issue, and her issue in her flesh be blood, she shall be put apart seven days: and whosoever toucheth her shall be unclean until the even.

And every thing that she lieth upon in her separation shall be unclean: every thing also that she sitteth upon shall be unclean.

And whosoever toucheth her bed shall wash his clothes, and bathe himself in water, and be unclean until the even.

And whosoever toucheth any thing that she sat upon shall wash his clothes, and bathe himself in water, and be unclean until the even.

And if it be on her bed, or on any thing whereon she sitteth, when he toucheth it, he shall be unclean until the even.

And if any man lie with her at all, and her flowers be upon him, he shall be unclean seven days; and all the bed whereon he lieth shall be unclean.

And if a woman have an issue of her blood many days out of the time of her separation, or if it run beyond the time of her separation; all the days of the issue of her uncleanness shall be as the days of her separation: she shall be unclean.

Every bed whereon she lieth all the days of her issue shall be unto her as the bed of her separation: and whatsoever she sitteth upon shall be unclean, as the uncleanness of her separation.

And whosoever toucheth those things shall be unclean, and shall wash his clothes, and bathe himself in water, and be unclean until the even.

But if she be cleansed of her issue, then she shall number to herself seven days, and after that she shall be clean.

And on the eighth day she shall take unto her two turtles, or two young pigeons, and bring them unto the priest, to the door of the tabernacle of the congregation.

And the priest shall offer the one for a sin offering, and the other for a burnt offering; and the priest shall make an atonement for her before the LORD for the issue of her uncleanness.

Thus shall ye separate the children of Israel from their uncleanness; that they die not in their uncleanness, when they defile my tabernacle that is among them.

This is the law of him that hath an issue, and of him whose seed goeth from him, and is defiled therewith;

And of her that is sick of her flowers, and of him that hath an issue, of the man, and of the woman, and of him that lieth with her that is unclean.