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Are only mature sperm able to fertilize an egg? If so, is there typically a reserve of mature sperm that are saved after an ejaculation or does a male need to wait for new sperm to mature before trying to fertilize an egg again? In humans it seems that the the shortest maturity period for sperm is around 42 days (New Findings on Sperm Life Cycle Could Impact Fertility Treatments). Do the new immature sperm serve any purpose after ejaculation?

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    $\begingroup$ Do you mean naturally? $\endgroup$ – WYSIWYG Dec 19 '16 at 19:34
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    $\begingroup$ @WYSIWYG, yes, not IVF. $\endgroup$ – Akromyk Dec 19 '16 at 19:35
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    $\begingroup$ There seems to be the misconception that sperm maturation period starts over after each ejaculation. It is a continuous process : at any given time, there will be populations of cells at various stage in the process; for instance, sperms that are at day 36 will be mature in 6 days, regardless of intervening ejaculations that could expend mature sperms. $\endgroup$ – Eliane B. May 21 '17 at 14:54
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As far as I know, sperm cells undergo maturation in the epididymis, where they become motile. As Eliane pointed out, this is a continuous process, so the male does not have to "wait", because there is continuously a batch of sperm cells that will shortly become mature. Furthermore, maternal factors are required for the process of capacitation in which the sperm cell becomes capable of fertilization and undergoes motility increase. Spermatozoa maturation and capacitation

An immature sperm cell is simply non-motile and even if present in the ejaculate, it will not be able to travel towards the fallopian tube to fertilize the oocyte. A sperm cell which did not go capacitation will also have no ability to fertilize an egg (that's why prior to IVF it's important to capacitate sperm).

My sources:

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