"Regression of the endometrium is usually accompained 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 and 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 endometrial regression that arises when there is too much blood and other tissue for complete reabsortion. The copious bleeding of humans and chimps can be attributed to the large size of the uterus relative to adult female size and to the design of the microvasculature in catarrhines."