Menstruation is a distinctive feature of human reproduction shared by apes, monkeys, a few bats – and sengis (also known as elephant-shrews).
How is this relevant to the evolving placenta? In addition to trophoblast and other fetal tissues, most placentas incorporate part of the endometrium (the non-muscular part of the uterine wall). It comprises connective tissue, glands and blood vessels. In preparation for pregnancy, the endometrium undergoes a process called decidualization. This involves a change in the size, shape and properties of the connective tissue cells. It is a necessary prerequisite for implantation of the blastocyst – an early stage in embryonic development.
In most mammals, decidualization does not occur until there is an embryonic signal. So there is a good chance the decidua will come in useful and help build a placenta. But in humans decidualization is spontaneous – in response to a maternal signal in the second half of the menstrual cycle. The decidua will be useful if there is a pregnancy, but otherwise must be shed by menstruation.
Why do women menstruate when mice do not? A recent paper by Emera, Romero & Wagner suggests there is an advantage to spontaneous decidualization. They believe early development of the decidua makes it easier for the mother to detect and reject defective embryos. Therefore natural selection has favoured the evolution of spontaneous decidualization. Menstruation is of no value in itself – just an inevitable corollary. The original paper develops the idea in much greater detail.
It makes even better sense if we consider the natural history of our species. Roger Short once took a careful look at the anthropological data, including studies of the Kung hunter-gatherers of the Kalahari. The first thing he noted was that women married young but it took a couple of years before they became pregnant. They started married life with infertile cycles. Teenage pregnancy was not a problem in primitive societies because it was a biological impossibility. At the other end of the scale, life span was briefer than in more advanced societies. Short reckoned a woman would manage about five pregnancies. Importantly, each of these children would be nursed for about three years. There are no fertile cycles during pregnancy and lactation and thus no menstruation. In total Short reckoned a female hunter-gatherer would experience 15 years of lactational amennorhoea, four years of pregnancy and just four years of menstruation. So menstruation was not such a big price to pay for the perceived advantages of spontaneous decidualization.
And sengis? They make a decidua with a cosy little chamber to welcome the embryo. If no pregnancy occurs, it is shed by a process akin to menstruation. The South African embryologist C. J. van der Horst documented this in the 1940’s. His work has been largely forgotten so it was nice to see it cited by Emera and colleagues.