The Evolution Of Endometrial Cycles And Menstruation Biology Essay

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According to a recent hypothesis, menstruation evolved to protect the uterus and oviducts from sperm-borne pathogens by dislodging infected endometrial tissue and delivering immune cells to the uterine cavity. This hypothesis predicts the following: (1) uterine pathogens should be more prevalent before menses than after menses, (2) in the life histories of females, the timing of menstruation should track pathogen burden, and (3) in primates, the copiousness of menstruation should increase with the promiscuity of the breeding system. I tested these predictions and they were not upheld by the evidence. I propose the alternative hypothesis that the uterine endometrium is shed/resorbed whenever implantation fails because cyclical regression and renewal is energetically less costly than maintaining the endometrium in the metabolically active state required for implantation. In the regressed state, oxygen consumption (per mg protein/h) in human endometria declines nearly sevenfold. The cyclicity in endometrial oxygen consumption is one component of the whole body cyclicity in metabolic rate caused by the action of the ovarian steroids on both endometrial and nonendometrial tissue. Metabolic rate is at least 7% lower, on average, during the follicular phase than during the luteal phase in women, which signifies an estimated energy savings of 53 MJ over four cycles, or nearly six days worth of food. Thus the menstrual cycle revs up and revs down, economizing on the energy costs of reproduction. This economy is greatest during the nonbreeding season and other periods of amenorrhea when the endometrium remains in a regressed state and ovarian cycling is absent for a prolonged period of time. Twelve months of amenorrhea save an estimated 130 MJ, or the energy required by one woman for nearly half a month. By helping females to maintain body mass, energy economy will promote female fitness in any environment in which fecundity and survivorship is constrained by the food supply. Endometrial economy may be of ancient evolutionary origin because similar reproductive structures, such as the oviducts of lizards, also regress when a fertilized egg is unlikely to be present. Regression of the endometrium is usually accompanied by reabsorption, but in some species as much as one third of the endometrial and vascular tissue is shed as the menses. Rather than having an adaptive basis in ecology or behavior, variation in the degree of menstrual bleeding in primates shows a striking correlation with phylogeny. The endometrial microvasculature is designed to provide the blood supply to the endometrium and the placenta, and external bleeding appears to be a side effect of endometerial regression that arises when there is too much blood and other tissue for complete reabsorption. The copious bleeding of humans and chimps can be attributed to the large size of the uterus relative to adult female body size and to the design of the microvasculature in catarrhines.

PMID: 8693059 [PubMed - indexed for MEDLINE]

Publication Types, MeSH Terms, Substances

Publication Types:


MeSH Terms:



Embryo Implantation


Energy Metabolism






Menstrual Cycle/physiology*


Models, Biological


Pan troglodytes







LinkOut - more resources

Ncbi pubmed

Q Rev Biol. 1993 Sep;68(3):335-86.

Menstruation as a defense against pathogens transported by sperm.

Profet M.

Division of Biochemistry and Molecular Biology, University of California Berkeley 94720.

Sperm are vectors of disease. During mammalian insemination bacteria from the male and female genitalia regularly cling to sperm tails and are transported to the uterus. I propose that menstruation functions to protect the uterus and oviducts from colonization by pathogens. Menstrual blood exerts mechanical pressure on uterine tissue, forcing it to shed, and delivers large numbers of immune cells throughout the uterine cavity, directly combating pathogens. The mechanisms of menstruation show evidence of adaptive design. Spiral arteries that open to the lining of the uterus trigger menstruation by abruptly constricting, which deprives the local tissue of blood, and then abruptly dilating, which causes blood to force loose the necrotic tissue. Menstrual blood flows easily, unlike blood at most wound sites, because it lacks the normal level of clotting factors. Overt (externally visible) or covert (not externally visible) menstruation has been documented in many species of primate, including Old World monkeys and apes, New World monkeys, and prosimians, as well as in various species of bat and insectivore. The antipathogen hypothesis predicts that: (1) menstruation (overt or covert) is either universal or nearly so among mammalian species; (2) if the latter, then the existence of menstruation among species varies inversely with the probability of becoming pregnant per estrous cycle (menstruation would be especially adaptive in species with significantly less than 100% probability of becoming pregnant per estrous cycle); (3) among menstruating species, the average degree of menstrual bleeding for a given species is a function of the factors affecting menstruation's costs and benefits--in particular, the degree of bleeding is positively correlated with the average body size and sexually transmitted pathogen load of that species (profuse bleeding would be especially adaptive in large-bodied species with either promiscuous breeding systems or continuous sexual receptivity); and (4) other forms of normal uterine bleeding--proestrous, periovulatory, implantation, and postpartum--also have an antipathogen function. The hypothesis presented in this article has implications for the diagnosis, treatment and prevention of uterine infection and, therefore, for the prevention of pathogen-induced infertility. The uterus appears to be designed to increase its bleeding if it detects infection: Human uteri that become infected (or otherwise inflamed) bleed more profusely, bleed on more days per cycle, and often bleed intermittently throughout the cycle. Thus artificially curtailing infection-induced uterine bleeding may be contraindicated.

PMID: 8210311 [PubMed - indexed for MEDLINE]


The adaptive evolution of bacteria, viruses, other microbes and parasites plays a central role in medicine since this process is needed to understand issues such as antibiotic resistance,[9] pathogen virulence.[10] and pathogen subversion of the immune system.[11]

Antibiotic resistance

Microorganisms evolve resistance through natural selection acting upon random mutation. Once a gene conferring resistance arises to counteract an antibiotic, not only can that bacteria thrive, but it can spread that gene to other types of bacteria through horizontal gene transfer of genetic information by plasmid exchange. It is unclear whether the genetic information responsible for antibiotic resistance typically arises from an actual mutation, or is already present in the gene pool of the population of the organism in question.[citation needed]

For more details on this topic, see antibiotic resistance


The effect of organisms upon their host can vary from being symbiotic commensals that are beneficial, to pathogens that reduce fitness. Many pathogens produce virulence factors that directly cause disease, or manipulate their host to allow them to thrive and spread. Since a pathogen's fitness is determined by its success in transmitting offspring to other hosts, it was thought at one time, that virulence moderated and it evolved toward commensality. However, this view is now questioned by Ewald.

For more details on this topic, see virulence, virulence factors and optimal virulence

Immune evasion

The success of any pathogen depends upon its ability to evade host immunity. Therefore, pathogens evolve methods that enable them to infect a host, and then evade detection and destruction by its immune system. These include hiding within host cells, within a protective capsule (as with M. tuberculosis), secreting compounds that misdirect the host's immune response, binding its antibodies, rapidly changing surface markers, or masking them with the host's own molecules.

For more details on this topic, see manipulation of the immune system by pathogens, and evasion of the innate immune system

Human adaptations

Adaptation works within constraints, makes compromises and tradeoffs, and occurs in the context of different forms of competition.[12]


Adaptations can only occur if they are evolvable. Some adaptations which would prevent ill health are therefore not possible.

DNA cannot be totally prevented from undergoing somatic replication corruption; this means that cancer, which is caused by somatic mutations, can never be completely eliminated by natural selection.

Humans cannot biosynthesize Vitamin C, and so risk scurvy, Vitamin C deficiency disease, if dietary intake of the vitamin is insufficient.

Retinal neurons and their axon output have evolved to be inside the layer of retinal pigment cells. This creates a constraint on the evolution of the visual system such that the optic nerve is forced to exit the retina through a point called the optic disc. This in turn creates a blind spot. More importantly, it makes vision vulnerable to increased pressure within the eye (glaucoma) since this cups and damages the optic nerve at this point, resulting in impaired vision.

Other constraints occur as the byproduct of adaptive innovations.

Trade-offs and conflicts

One constraint upon selection is that different adaptations can conflict, which requires a compromise between them to ensure an optimal cost-benefit tradeoff.

Running efficiency in women, and birth canal size[13]

Encephalization, and gut size [14]

Skin pigmentation protection from UV, and the skin synthesis of Vitamin D

Speech and its use of a descended larynx, and increased risk of choking[15]

Competition effects

Different forms of competition exist and these can shape the processes of genetic change.

mate choice and disease susceptibility [16]

genomic conflict between mother and fetus that results in pre-eclampsia[17][18]

MHC mate choice[19][20]‎

Maternal-paternal genetic competition that by altering genetic imprinting might underlie autism and schizophrenia[21]

Evolved defense mechanisms

Evolution has selected defense mechanisms that protect against injuries and infections.[8] These include








Morning sickness




Sickness behavior (Lethargy, Depression, Anorexia, Sleepiness, reduction in grooming, and failure to concentrate)

Expulsions: Sneezing, Vomiting, Coughing, Diarrhea

5.3.1 Define pathogen.

Pathogen - an organism or virus that causes a disease.

5.3.2 State one example of a disease caused by members of each of the following groups: viruses, bacteria, fungi, protozoa, flatworms and roundworms.


Bacteria: Cholera.

Fungi: Athlete's foot.

Protozoa: Malaria.

Roundworms: Ascaris eggs contained in contaminated food are swallowed, circulate through the blood stream, reach the lungs, grow into larvae in the nasal cavities, swallowed into the stomach where they grow into adult worms and start the cycle again.

Flatworms: Pork tapeworm.

5.3.3 List six methods by which pathogens are transmitted and gain entry to the body.

1) From the air, 2) Direct contact, 3) Through food, 4) Cuts in the skin, 5) Blood transfusion, 6) Animals and insects.

5.3.4 Describe the cause, transmission and effects of one human bacterial disease.

Diptheria is a bacterial disease the is breathed in and infects the nose, throat, and larynx. The bacteria releases toxins that destry tissues in the heart nerves and glands.

5.3.5 Explain why antibiotics are effective against bacteria but not viruses.

Antibiotics block specific metabolic pathways found in bacteria, but not in eukaryotic cells. Viruses reproduce using the host cell metabolic pathways that are not affected by antibiotics.

5.3.6 Explain the cause, transmission and social implications of AIDS.

AIDS is a retrovirus having RNA as its genetic material and not DNA. It transcribes its RNA into DNA using an enzyme called reverse transcriptase. IDS is a syndrome where the immun system fails and opportunistic pathogens cause further harm. It is transmitted by sexual intercourse, sharing of needles, blood transfusions, accidents causing blood contamination, cuts in the skin, tattoos and ear piercing with infected needles. Social implications are that people don't feel very comfortable with a person who has AIDS. People with AIDS can find it difficult to buy health insurance plans, find jobs, have friends, and build normal social relations. People have changed their sexual life styles due to awareness and education about AIDS.

Topic 5.4 - Defense Against Infectious Disease


5.4.1 Explain how skin and mucous membranes act as barriers against pathogens.

The skin and mucous membranes are the first lines of defense against disease. The skin has a thick keratin layer on the surface which doesn't allow any organisms to enter the body. Where there is no skin, such as the mouth cavity, epithelial cells there form a mucous membrane that produces mucous which traps and stops the action of many pathogens.

5.4.2 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.

When the phagocytes meet the pathogens, they ingest the organisms by phagocytosis (eating). Once they are in the phagocytes, the pathogens go into the vesicles which fuse with the lysosomes, which then release hydrolytic enzymes on them and destroy them.

5.4.3 State the difference between anitgens and antibodies.

An antigen is a foreign macromolecule that does not belong to the host organism and that elicits an immune resonse. An antibody is a protein and is called an immunoglobulin. It is made of 4 polypeptides, 2 heavy chains and 2 light chains. It sticks to antigens and to lymphocytes.

5.4.4 Explain antibody production.

Many different types of lympocytes exist. Each type recognizes one specific antigen and responds by dividing to form a clone. This clone then secretes a specific antibody agaist the antigen.

5.4.5 Outline the effects of HIV on the immune system.

HIV attacks helper T cells, which are part of the immune system that are important for the function of B lymphocytes. The virus enters the helper T cells and replicates there. The cells burst and release new viruses, these viruses infect other helper T cells and possibly other cells such as phagocytes as well. The destruction of helper T cells paralyses the immune system since they communicate between different cells of the immune system and activate them. This enables any other parasite or organism usually kept under control by the immune system to be able to affect the body. What makes this disease more serious than others is that HIV replicates in a immune system cell. Therefore, by creating more of itself it is also killingthe cells that could kill it.

According to Coutinho's definition, menstruation is simply the sign of a failed process: "When menstruation occurs, it means that the [reproductive] system failed and, for the sake of reproductive efficiency, would have to be repeated the next month, the month after that, and so on, until a successfully nested fertilized egg starts to develop" (4). This is not far from the standard definition of menstruation provided in health education classes: menstruation takes place when pregnancy does not.

Coutinho's definition of menstruation is important since it underpins his major claim in this new work -- that regular menstruation is not "natural." According to Coutinho, a monthly menses would have been unusual for early women who were regularly pregnant or breast-feeding (and therefore without periods), "young women were either pregnant or lactating almost continuously" (2). It is only the modern woman, he argues, who experiences menstruation as a regular, monthly occurrence. While repeated menstruation made biological sense for Stone Age humans whose survival was by no means assured, Coutinho hypothesizes, regular menstruation is no longer necessary in the modern world where human survival is not contingent upon prolific childbirth.

Coutinho concludes with a syllogistic logic: since menstruation exists for the purpose of prolific childbearing, and repeated childbirth is no longer necessary, then menstruation is now "obsolete". Without the promise of 10 or 12 children to bear, menstruation, according to Coutinho, is a waste of a woman's resources. It takes away her energy, lowers her iron levels and induces an array of minor health troubles -- headaches, nausea, cramps, moodiness -- and major health symptoms for those with chronic menstrual ailments, such as endometriosis. Regular menstruation, Coutinho concludes, is an outmoded function of our evolutionary ancestors and should now be suppressed in all reproductive aged women.

Not all scientists, however, are so quick to dismiss the import of women's monthly bleeding. Margie Profet -- a young, maverick evolutionary biologist from the University of California, Berkeley -- made her entry into the scientific forum in 1993 by asking a question no scientist had thought to ask since Hippocrates and Galen: "Why do women menstruate?" Profet's findings, painstakingly detailed in an article for the renowned Quarterly Review of Biology, come remarkably close to her scientific forbears. Profet argues from an evolutionary standpoint that there must necessarily exist a functional purpose for regular menstruation or it would not have endured the mutations of our evolution. Menstruation must offer some advantage for human survival or it would not have survived itself. It is not likely, Profet maintains, that our bodies are so inefficient as to permit a monthly expenditure of energy without a concurrent gain.

Profet noted at the outset that menstrual blood differs in composition from that of regular blood, most notably by containing immune cells called "macrophages". These cells are able to combat the presence of pathogens present in the uterine cavity. It is from this observation that Profet establishes her hypotheses: "Menstruation functions to protect the uterus and oviducts from colonization by pathogens" (335). Regular bleeding is a regular cleansing, in Profet's estimation, keeping women's reproductive organs free of contaminants. And from where do these pathogens come? From men, of course: "Sperm are vectors of disease" states Profet unhesitatingly (335). Sexually active women require a method by which to protect themselves from potential infection caused through intercourse. Menstruation is nothing less than a sign of the ongoing war of the sexes -- the natural means through which sexually active, heterosexual women protect themselves from men.

The enforced cessation of menses then, from Profet's perspective, would be harmful to a woman's health rather than beneficial. "The uterus appears to be designed to increase its bleeding if it detects infection….Thus artificially curtailing infection-induced uterine bleeding may be contraindicated" -- since it interferes with her body's natural capacity to defend itself against pathogens (355).

Critics of Profet, of which there are many, argue to the contrary that menstrual blood acts as the perfect nesting ground for a host of sexually transmitted microorganisms, and moreover, a woman is more susceptible to a wide variety of vaginal infections during menstruation than at any other time in her cycle. Profet accepts the fact that some microorganisms flourish during menstruation, but notes that while humans have evolved to maximize survival, so have pathogens. The continued threat of sexually transmitted disease only highlights the fact that our evolutionary battle with bacteria is never over.

A recent book by Village Voice reporter, Karen Houppert (The Curse: The Last Unmentionable Taboo) adds another dimension to the menstruation debate. Houppert collates health studies conducted on toxic shock syndrome and other reproductive health problems (including infertility and endometriosis) and finds that much of these ailments may be directly caused by even trace levels of dioxins found in most tampons and pads --the chlorine compounds that make our "sanitary protection" whiter than white. Given Houppert's findings, it may be less that menstruation "causes" the onset of infectious diseases, as critics of Profet claim, than that our "treatment" of menstruation interferes with a natural immune process.

Beverly Strassmann, an anthropologist from the University of Michigan, challenged Profet's hypothesis in a subsequent (1996) article from the same journal (Quarterly Review of Biology) arguing that Profet has more in common with her critics than one would first suppose. Profet, like Coutinho, and most researchers of menstruation focus their attentions on the physical act of expelling blood from the vagina. Strassmann to the contrary 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? Like Profet, she too 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 that the "endometrial 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, 181).

While scientists continue to engage in the debate over the functional attributes of menstruation and their value in the modern world, feminist critics can assess Coutinho's proposal from other angles. Firstly, there are questions concerning the accuracy of Coutinho's perpetually pregnant ancient woman. We have little evidence to pronounce conclusively that women rarely menstruated in the past. Rather, we have ample evidence to suppose that women were regularly practicing birth control methods (and hence, menstruating) in countless cultures (O'Grady, 2000). Menstrual rituals and ceremonies, as well as menstrual accoutrements (early versions of pads and tampons) are also detailed by multiple historical documents from cultures across the globe ([1] Even if Coutinho could prove conclusively that repeated pregnancy existed for the ancient woman, he neglects to note that this would not describe a woman's "natural state" (a condition imposed by nature) but, rather a social and environmental condition (as modern woman's decision to have one or two children is socially and environmentally dictated).

For scholars like Emily Martin (author of Woman in the Body) what Coutinho's text presents is an example of the normative paradigms that continue to function in scientific discourse, particularly in relation to women's bodies which are seen as aberrations from a male "norm". "Menstruation," in Coutinho's understanding, is "unnatural," that is, "pathological," a "sickness" that the medical establishment must labor to cure. Science may be working rapidly to perfect the human body, but it is certainly not shaping it in the image of woman.

In the end it may not be so surprising to find that the means through which Coutinho suggests that menstrual suppression can be achieved is via regular Depo-Provera injections, the birth control method that he, himself, pioneered. So while the media continues to herald Coutinho's discovery that menstruation is not "natural" and is an ailment that has a ready cure, good feminist studies of menstruation and menstrual history, of which there are many, indicate that such a pronouncement is suspect at best. What is required are independent assessments of Coutinho's work (from those not likely to benefit directly from the wide-spread usage of Depo-Provera). Ideally these studies would begin from a premise contrary to Coutinho - that menstruation is "natural," that it has a purpose, and that its function may not be limited to potential pregnancy. Only when we can first assess the value of the regular processes of women's bodies can we fully understand their role and function in the physical and emotional health of all women.

Sperm mediated transfer of HIV into target cells: role in AIDS transmission.

Miller VE, Scofield VL; International Conference on AIDS.

Int Conf AIDS. 1989 Jun 4-9; 5: 514 (abstract no. M.C.O.29).

Dept. Microbiology and Immunology, UCLA, Los Angeles, California 90024 USA

We have previously reported that human sperm bind to and penetrate human lymphocytes. This interaction is mediated by the HLA-DR and -DQ ligands on target cells and a CD4-like receptor on sperm. We have identified two mechanisms of sperm entry into lymphocytes and HLA-DR transfected murine fibroblasts: 1) endocytosis, where electron micrographs show sperm clearly inside endocytic vesicles, and 2) direct fusion between sperm and target cell membranes, in which case sperm heads are visible inside the cytoplasm with no surrounding membrane. We have subsequently shown that normal sperm, when preincubated with HIV, bind to and penetrate CD4+, HLA-DR- cells, and that in this case, the ligand on the target cells is the CD4 molecule. We have proposed that the event of somatic cell penetration by sperm is one mechanism for AIDS transmission by semen. According to this model, HIV transported by sperm would have access to non-CD4+ cells, and if penetration is by direct fusion, the virus would be directly exposed to the cytoplasm. In support of this model, we have evidence that sperm isolated from AIDS patients is capable of infecting activated PBL cultures in vitro. Two different separation techniques were used: 1) the swim-up method, where motile sperm are allowed to swim-up from a pellet into culture medium, and 2) centrifugation through 90% Percoll. Although a substantial number of round cells (immature germ cells) were visible in some of these preparations, staining with anti-CD45 antibodies revealed no lymphocyte contamination. In addition, using immunogold-electron microscopy we detected the presence of gp120 antigens on AIDS patients' sperm cell surfaces. We are currently investigating the possibility of HIV transmission by normal donor sperm and HIV combinations.