Why do men produce so much spermatozoa, that will get discarded, but women, just one ovulum, which is a good one.
Couldn't men produce good spermatozoa as a limited edition?
Where is the evolutionary advantage of things being like they are?
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$\begingroup$ The way which the spermatozoa have to travel is long and relatively dangerous. So you want to make sure that at least one is reaching its final goal. Having many ovums would mean that you get more than one embryo... $\endgroup$– Chris ♦Commented Mar 10, 2014 at 19:06
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$\begingroup$ There surely some room for lower spermatozoa concentrations. But since there are reports about fertility problems in men with lower spermatozoa counts, I doubt there is much room. $\endgroup$– Chris ♦Commented Mar 10, 2014 at 19:18
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3 Answers
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I think the asymmetry allows for a stronger selection process. Sperm competition is a powerful force in evolution.
This is useful since sperm vary wildly in quality - for healthy human males only 50% might not be normally motile. Other animals can have a higher percentage, but there are always a significant number of floaters... This means that the fertilization via sperm takes out a lot of the less functional sperm. Since these are in many cases the same genes we use as diploid organisms later, sperm competition really tames a lot of problems which might be produced in meiosis.
Many animals have multiple mating events from multiple males at just about the same time - when the female is in heat. This gives a larger role for sperm competition - literally different sires are competing as sperm to fertilize the egg.
In females it is a different process of choosing the cells. Ova are mostly eliminated in women by the time they reach reproductive age. I think that this is not well understood biologically... references are hard to find.
The differences in the morphology and roles of the gametes, which goes back to the most primitive of animals has a strong role in selection of healthy offspring through a diverse set of mechanical and biochemical conditions experienced during fertilization.
Added Note: I ran across this reference for how ova are selected for presentation by the ovaries. Its not really a review, but maybe a lead reference on how selection works in the female reproductive system - its also picky:
In mammals, ovarian folliculogenesis is a process regulated by complex networks composed of various endocrine, paracrine and autocrine factors that interact coordinately.
Its no cake walk to being an egg either...
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1$\begingroup$ In addition to motility - up to 70% of sperm can be abnormal forms and this is still considered normal. $\endgroup$– Rory MCommented Mar 11, 2014 at 12:02
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$\begingroup$ that's interesting! I wonder if they end up fertilizing the egg? $\endgroup$– shigetaCommented Mar 11, 2014 at 16:38
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$\begingroup$ morphology isn't always fitness, though for diploid human beings it seems to be important. $\endgroup$– shigetaCommented Mar 12, 2014 at 0:27
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Shigeta's answer is excellent. I also agree with Chris in his discussion of ova, upon which I want to expand.
This is in part speculation, but I believe that the answer to producing (usually) only one ovum lies in the development of animal viviparity.
Many animals (especially aquatic animals) currently and historically have reproduced via oviparity - the female producing a multitude of ova and allowing the male to shower them with spermatozoa. The general style of males producing a large number of spermatozoa seems to not have changed greatly (aside from location of sperm deposition) through evolution.
However, viviparous animals must by necessity have some sort of limit on the number of simultaneous offspring, as the females carry offspring within their own bodies.
In animals who reproduce with hemotrophic viviparity, the embryos are provided nutrients from the mother through a placental organ that interfaces with the maternal bloodstream; the biological cost of having many offspring depends on how long they are carried and how large they are at term.
Most mammals carry a litter and develop small, underdeveloped young, but hominids evolved to carry (usually) one. There's an interesting discussion to be had on the different reproductive strategies (very careful rearing of one versus attention divided over many) but I don't have sources at the moment for those.
But the cost of having more than one embryo must have been significant enough in our own past evolutionary process to have eliminated most women who produced multiple ova. Maternal mortality rates have historically been MUCH higher than they are now, and risk is higher with twins (or more). There are still times when more than one ovum reaches maturation and ovulates, resulting in dizygotic twins, but it's fairly rare (between 6 and 14 per thousand births).
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As spermatozoa need to be transported therefore in order to increase the chances of successful transportation (and fertilization) the number may be high.
Also, there is only one ovum probably because more cytoplasm is required in the creation of an ovum, which is evident from its size.
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$\begingroup$ i like the idea of the sperm are small and easy to produce and eggs are large and difficult to produce idea it seems relevant to the nature of us as a species rather than a pat quotation of evolution. $\endgroup$– user1357Commented Apr 27, 2014 at 3:23