How does not recycling the endometrial lining of the uterus benefit humans (and other organisms that menstruate), while recycling it is beneficial to horses (and other organisms that undergo estrus cycle)?

So the question linked by user137 gave me some useful points to work from. Bear with me though, because I learned alot of new jargon just by reading it.

Helps immunity:

  • "forces loose not only the endometrial lining but sperm-borne pathogens contained therein."

  • "menstruation delivers immune cells to the uterine cavity, triggered by the mechanical pressure exerted via the shedding of endometrial lining."

Energy efficient:

  • its simply more energy efficient to shed. saving 6 days worth of food per 4 cycles.

  • for individuals/species with high fecundability shedding of the lining is less useful

  • larger uterus = increased likelihood of menstrual shedding (from the answer I learned humans have a larger uterus -- relative to body size -- compared with other mammals who don't shed.)

Miscarriage discardation?:

the comment section also had an interesting reason saying that sloughing off the lining regularly helps get rid of dead embryos.

All of this is very useful and interesting but I'm still confused as to why horses don't shed. I would think horses have low fecundability since their awkward bodies would make it hard to mate. And if shedding the lining is more energy conservant, then why don't horses? I want to know why it is evolutionarily better for horses to fully reabsorb, compared to humans?

  • $\begingroup$ What do you mean with help? $\endgroup$
    – Ebbinghaus
    Commented Jun 8, 2016 at 9:26
  • 3
    $\begingroup$ I don't have the source right now, but human women do reabsorb much of their uterine lining during menstruation. However, there is so much of it, and the duration of menstruation is so short, that it can't all be reabsorbed in time, so it leaks out instead. EDIT: here is a related question that might be more informative. $\endgroup$
    – user137
    Commented Jun 8, 2016 at 10:09
  • $\begingroup$ @JordiZambrino When I say help im assuming its better for human females to do it this way and i'm wondering why. $\endgroup$
    – user24362
    Commented Jun 12, 2016 at 1:20
  • $\begingroup$ @user137 thanks but that source doesnt say anything about humans reabsorbing their lining . at least nothing that I could find. it really useful though thankyou $\endgroup$
    – user24362
    Commented Jun 12, 2016 at 1:24
  • $\begingroup$ Horses do not undergo menstrual cycle; they have estrus. $\endgroup$
    Commented Jun 20, 2016 at 6:30


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