Primate Fertility

While the reasons behind why women menstruate remain unclear, research shows that the number of menstrual cycles modern women experience differs greatly from the number experienced by pre-agricultural women. It is impossible to know with certainty the reproductive patterns that prevailed 10,000 years ago. However, it is likely that the reproductive patterns of Stone Age women are more closely related to those of current hunter-gatherer societies than to those of western women (Eaton and Eaton III 1999). The best opportunity to study the natural pattern of human reproduction occurs with women in current foraging societies. American women currently experience three times as many menstrual periods as women who have continued living in the ways of earlier ancestors. Foraging women are 16 years old at menarche, 19.5 year old at first birth, nurse for three to four years, have a completed family size of 5.9 live births, and an average age at menopause of 47 years. They experience a total of 160 ovulations in their lifetime. Contemporary American women are 12.5 years old at menarche, 24 years old at age of first birth, nurse for 3 months (if at all), have a completed family size of 1.8, and are 50.5 years old at menopause. American women experience approximately 450 ovulations within their lifetime (Eaton et al 1994). A study done with the Dogon women of Mali shows a similar relationship. The Dogon are a foraging society that practices natural fertility by not using modern contraceptive methods. The Dogon have a fertility rate of 8.6 ± 0.3 live births per woman. Median number of lifetime menses experienced by the Dogon was 109, with a U-shaped relationship between menstruation and age showing that, from menarche to menopause, women in primary child-bearing years (20 —34 years old) rarely menstruated (Strassman 1999). Overall, this data indicates that monthly menstruation for decades on end is not the historical norm. Today, women have earlier menarche, later first birth, and fewer pregnancies. There is also a decreased suppression of menstruation through lactation as _ of children are never breast-fed and the rest only breast-feed for 3 months. Early menarche is an especially recent development. In the 19th century, the age of first menarche was the same as in the hunter-gatherer women observed today. The earlier age of first menarche can be linked to an increase in caloric intake, while at the same time occupational, educational, and social forces have led to a later first birth (Eaton and Eaton III 1999). The consequences for these changes in menstrual cycling may be seen in cancer rates among women in industrialized nations.

The increased menstruation of modern, urban women has to be related to their diet.

The increased menstruation of modern, urban women has to be related to their diet.

I would think it's an evolutionary advantage for all females of every type (of mammal) to be fertile as much of the time as possible, so evolution probably pushes things in this direction. So there must be some selective mechanism in other mammals which presents a downside to that.