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Comment by jessaustin | Hacker News Reader

Mice and rats are an interesting case, since they have hemochorial placentae but not menstruation. This somewhat undercuts TFA's "this because of that" narrative. (Really the only flaw in a wonderful article.) Evolution is weird.

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Apparently some mice do menstruate[1]. Do mice have another mechanism for handling the issue the author raises: "What to do when the embryo died or was stuck half-alive in the uterus?"

[1]: https://www.nature.com/news/first-rodent-found-with-a-human-...

I would speculate that it works similarly to most other mammals. Spikes of prostaglandin in the blood cause abortion via smooth muscle action and fluid production (or, if the embryo has developed to term, parturition rather than abortion). Prostaglandin is produced in the body cyclically. The corpus luteum releases progesterone that counteracts prostaglandin, but toward the end of the estrous cycle the CL degrades into an inert corpus albicans. Healthy implanted embryos release gonadotropin, which "stops the clock" for the CL so that it continues to produce progesterone. Eventually the placenta starts producing its own progesterone, and at that point the CL is no longer important. A dead or half-alive embryo is likely to produce neither gonadotropin nor progesterone, so it will be aborted and flushed on the next prostaglandin spike. The later this occurs in the pregnancy, the more stress is placed on the mother.

It's my impression that after implantation, this process is the same in humans as in other placental mammals. Thus menstruation might be more properly understood to be about flushing unimplanted zygotes rather than aborting unhealthy embryos.

> This somewhat undercuts TFA's "this because of that" narrative. (Really the only flaw in a wonderful article.)

You're underselling this flaw, because it's fatal. A major issue with a lot of pop science is this exactly: the author is much too confident in connecting disparate ideas (because it serves good storytelling and increases trust in the author), when in actuality it's very dangerous to assign intent to things like evolution. For example, a common statement you hear is "evolution caused..." Evolution doesn't have a will or do anything deliberately, nor does it always tend towards the best way to do something or a beneficial mutation. The author also engaged in this flaw by giving fetuses wills (similar to how some writers anthropomorphize animals including primates). So while the doctor goes on in the whole post comparing different species and their different gestational evolution, at the end of the day, it's just a theory that combines different points.

Edit: Downboats? Berry?! Not on my HN!

> The quotes are to indicate that this isn't about what they consciously want, but about what evolution tends to optimize.

Sounds like the author tries to address your point.

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