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.
"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."
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.)
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?