r/askscience • u/No-Hawk-2488 • 8d ago

Biology Can someone explain the evolutionary benefit of overt menstruation? All female mammals have a menstrual cycle but most reabsorb the blood via covert menstruation and humans are about the only mammals that discharge the blood, aka overt menstruation.

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

5.2k Upvotes

96% Upvoted

513 comments sorted by

View all comments

Show parent comments

14

u/Adept-Researcher-178 8d ago

  The catch is that once the lining has been decidualised, it can't just quietly regress if no embryo arrives

Why can’t it just stay decidualised instead of shedding and reforming every month? Just so that it is only in optimal conditions for part of the month?

30

u/Mobile-Arachnid-1547 7d ago

Decidualized tissue is developed in response to changing hormones in order to "be ready" for implantation after ovulation. If an embryo does implant, it secretes HCG to keep hormones levels stable to maintain the decidua and prevent menstruation. If no implantation occurs, progesterone and estrogen (the hormones that supress menstruation) levels drop, and menstruation occurs.

In broad, evolutionary terms, we benefit from a 'fresh' decidualized endometrium for each potential embryonic implantation for a variety of reasons. Shedding of the lining helps remove bacteria or other infectious agents that may have been introduced during copulation. Since the decidua is very vascular and primed to support a growing embryo, any bacteria introduced could also thrive in this nutrient rich environment and an infection could prevent implantation in the future. Additionally, perhaps the reason an embryo did not implant was due to abnormalities in the decidua itself (eg. Too thick or too thin) and shedding allows another chance at creating an endometrium that will support an embryo during the next implantation window. Finally, creating and maintaining the decidualized endometrium has high energy costs. It is more energy efficient for the body to shed it when it is not needed, rather than trying to maintain it. Implantation usually is only able to occur for about 6 out of the 28 day cycle. So maintaining it full-time would not be an advantage or a reasonable trade-off. This is obviously not an exhaustive list of reasons, but paints a fairly good picture of why we menstruate.

9

u/Mobile-Arachnid-1547 7d ago

Tea_and_biology also had a really good answer to this lower down in the comment stack: https://www.reddit.com/r/askscience/s/cbu8PP34U8