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I was wondering why woman have fertile periods. Based on some simple reasoning I would expect that if woman were always fertile this would increase the chance of reproduction(one of the important things in life). But having menstrual periods would decrease this chance because only the period close to ovulation would result in fertilization.

So why did evolution result in woman having menstrual cycles?

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Human female ovaries only have a certain number of ova; none are manufactured in adulthood. Once they're gone (that's when menopause occurs), they're gone.

You can also ask, why did evolution result in a limited number of oocytes then? This kind of second guessing of physiology is endless.

In your scenario, an ovum would need to be released every few days in order to be fertile continuously. That alone would decrease the number of years that a woman could be fertile. Since a fetus needs about nine months to gestate, I think once a month fertility is more than adequate. The earth's population is already too large, which also translates as successful reproductively.

If a fetus could gestate in a month, I could see an advantage to constant fertility. But the disadvantages would also be great. How could a woman take care of so many babies? How could she possibly feed, say, six to nine babies a year with only two breasts? (Mammals that frequently have larger litters have more mammary glands.) How could her own body compensate nutritionally not only for the burden of feeding the fetuses but also of the babies?

Asking why didn't something evolve from a teleological standpoint rarely results in a satisfying answer. If constant fertility were advantageous, maybe it would be present.

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  • $\begingroup$ But there are animals which don't have these cycles, so probably there should be another reason why we humans (and some other animals) got these cycles and others did not $\endgroup$ – KingBoomie Nov 19 '16 at 15:09
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    $\begingroup$ @RickBeeloo - Are there mammals who don't have estrus cycles? Do these other animals physically care for their young through to near adolescence (age of "adolescence varies from species to species)? $\endgroup$ – anongoodnurse Nov 19 '16 at 15:16
  • $\begingroup$ Humans don't have estrous cycles, they have menstrual cycles (en.wikipedia.org/wiki/… ) so yes there are definitely mammals which don't have estrous cycles which is another good point in evolution (why menstrual cycles and not estrous cycles or visa versa) @anongoodnurse $\endgroup$ – KingBoomie Nov 19 '16 at 17:19
  • $\begingroup$ @RickBeeloo - Did I say humans had estrous cycles? I asked if you knew of any mammals that didn't have estrous cycles. You said, "But there are animals which don't have these cycles...". Since insects are actually animals, I know with great certainty that there are animals without estrous cycles. What you don't know doesn't make your argument more effective. There is no significant difference between menstrual and estrous cycles reproductively. One bleeds the excess endometrial tissue, the other reabsorbs it. Resorption may help avoid predation. $\endgroup$ – anongoodnurse Nov 19 '16 at 17:56
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    $\begingroup$ Reread my comment (which you did not answer); your question most certainly was not why do some mammals menstruate while others have estrus cycles (that's not "the whole point".) $\endgroup$ – anongoodnurse Nov 19 '16 at 18:09

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