Very few mammals menstruate, why?

Menstruation, while fundamental for human reproduction, remains a rarity in the animal kingdom. Why have some species evolved to undergo menstruation while the majority have not?


Among the few mammals that have a menstrual cycle, we primarily find primates like chimpanzees and bonobos, as well as a few species like certain bats, elephant shrews, and spiny mice. This phenomenon, which appears to have evolved independently in these species, could suggest that it provides a significant evolutionary advantage.

Unlike animals that bleed during estrus, menstruating species undergo complex hormonal changes involving not only estrogen but also progesterone. These hormones prepare the uterus for a potential pregnancy by thickening the uterine lining and developing new blood vessels. If fertilization does not occur, this preparation is then expelled in the form of menstruation.

One hypothesis proposed to explain this phenomenon is that these animals, which invest a lot of energy in a small number of offspring, have developed a system to optimize the survival chances of their descendants. The body might be able to detect early on if an embryo is viable, thereby avoiding an unnecessary investment in a pregnancy doomed to fail.

Another theory suggests that menstruation could play a role in eliminating aging sperm. In some species like bats, sperm can remain in the female's body for months before fertilization. The menstrual cycle could thus help eliminate degraded sperm, favoring fertilization by more robust sperm.

Despite these hypotheses, the mystery of menstruation remains partially unresolved. Further research is needed to fully understand why this unique adaptation has developed in some species and not in others.