THE EVOLUTION OF ENDOMETRIAL CYCLES AND MENSTRUATION

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 b e more prevalent before menses than after menses, (2) in the life life histories of females, the timing of menstruation should track pathoge n burden, and (3) in primates, the copiousness of menstruation should increase with the promiscuity of the breeding system. I tested these p redictions and they were not upheld by the evidence. I propose tile al ternative hypothesis that the uterine endometrium is shed/resorbed whe never implantation fails because cyclical regression and renewal is en ergetically less costly than maintaining the endometrium in the metabo lically active state required for implantation. In the regressed state , oxygen consumption (per mg protein/h) in hu,nan endometria declines nearly sevenfold. Tile 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 nonendometr ial tissue. Metabolic rate is at least 7% lower, on average, during th e follicular phase than during the luteal phase in women, which signif ies an estimated energy savings of 53 MJ over four cycles, or nearly s ix days worth of food. Thus, the menstrual cycle revs up and revs down , economizing on the energy costs of reproduction. This economy is gre atest during the nonbreeding season and other periods of amenorrhea wh en the endometrium, remains in a regressed state and ovarian cycling i s absent for a prolonged period of lime. Twelve months of amenorrhea s ave an estimated 130 MJ, or the energy required by one woman for nearl y half a month. By helping females to maintain body mass, energy econo my will promote female fitness in any environment in which fecundity a nd survivorship is constrained by the food supply. Endometrial economy may be of ancient evolutionary origin because similar reproductive st ructures, such as the oviducts of lizards, also regress when a fertili zed egg is unlikely to be present. Regression of the endometrium is us ually accompanied by reabsorption, but in some species as much as one third of the endometrial and vascular tissue is sited as the menses. R ather than having an adaptive basis in ecology or behavior, variation in the degree of menstrual bleeding in primates shows a striking corre lation with phylogeny. The endometrial microvasculature is designed to provide the blood supply to the endometrium and tile placenta, and ex ternal bleeding appears io ire a side effect of endometrial regression that arises when there is too much blood and other tissue for complet e reabsorption. Tire copious bleeding of humans and chimps can be attr ibuted ta tire large she of tire uterus relative to adult female body size and to the design of the microvasculature in catarrhines.