7
$\begingroup$

So apparently we produce roughly 300 million sperm daily. Is there a reason why this is necessary? Wouldn't a much smaller (but still large) number be sufficient? Like, one million?

$\endgroup$
  • 1
    $\begingroup$ Can you add a source for this number? $\endgroup$ – Chris Jul 22 '15 at 7:52
  • $\begingroup$ @Chris it's something they just taught us in the biology class (I imagine it is also in the book amazon.com/Biological-Science-MasteringBiology-3rd-Edition/dp/…). Google searches yield ranges from 80 to 250 million - either of these numbers serve the purpose of my question though. I don't really care how many are produced, but rather, what's the point of making so many. $\endgroup$ – Voldemort Jul 22 '15 at 8:04
  • 2
    $\begingroup$ The one winner out of millions is more special than the one out of two :) $\endgroup$ – AliceD Jul 22 '15 at 8:45
  • 2
    $\begingroup$ It's interesting that you think 300 million is "so many" but 1 million is not. My point is, one's intuition for what is a "reasonable" number of things in nature is often a poor guide. $\endgroup$ – Chelonian Jul 22 '15 at 16:14
  • 1
    $\begingroup$ @Voldemort Fair enough. My point about our intuition not being a good guide to what the numbers for things like this ought to be still seems applicable to human intuition generally; that is, why should 1 million, for example, be enough? Why not 24,500? The point is, the numbers are what they are, and maybe 300 million is the appropriate number (or maybe the system isn't optimized?). $\endgroup$ – Chelonian Jul 25 '15 at 3:23
5
$\begingroup$

The term you should look up is sperm competition, there is an introductory wikipedia article about it here, where you can get the basic picture and get some references to explore.

Simply said, males often compete with other males in the reproductive tract of the female, thus a higher number of sperm will lead to a higher chance of fertilizing the female in competition with other males - leading to an arms-race of the number of sperms a male eject into the female.

There is a lot more to it, of course, but I believe this well explains the most fundamental driver of why males generally produce and eject so many sperms.

$\endgroup$
  • $\begingroup$ If a male mates with the female, and then comes another male, wouldn't the first male be much more likely to succeed because he introduced the sperm beforehand? I imagine that just a few hours are a huge advantage, so I'm not sure why would one need so many millions of sperm anyway. $\endgroup$ – Voldemort Jul 23 '15 at 20:14
  • $\begingroup$ @Voldemort I think it's safe to say that any advantage of being first for a certain amount of time would be highly dependent on the nature of the species in question. But if the production of sperm is relatively cheap and the competition is relatively significant, I think it's easy to see that we might reach an arms-race here, even if the amount of sperm would not always have to be the most important factor. $\endgroup$ – Alex Jul 24 '15 at 10:43
  • 1
    $\begingroup$ @Voldemort And if we look at humans as an example, we are curiously unique in an unexpected way as our penises seem to have evolved to displace semen of other males from the vagina, so sperm competition seems to be very real even in humans. $\endgroup$ – Alex Jul 24 '15 at 10:48
  • $\begingroup$ Great reference Alex, right on track. Also high production of sperm also has to do with male health and dominance. In the wild a more dominant healthy male will produce more sperm and healthier sperm. Females pick up on this and will want to mate with someone strong. I'm humans this characteristic is not as widely perceived but women will tend to be attracted to a male that seams strong because psychologically he can " protect her and their young" of course we have evolved to money being a statement of power and dominance thus this can also serve as competition. All in all it boils down to ove $\endgroup$ – Mr Hons Apr 9 '16 at 23:16
2
$\begingroup$

The answer, if you search for a bit, boils down to geometry. It takes X amount of sperm cells to successfully fertilize an egg. If you optimize delivery system (e.g. vagina geometry), you'll need less. In species where one male fertilize number of eggs (aka polygamy) and are able to have progeny from number of females, you need less cells in ejaculate to produce single child, because chances are higher with higher number of matings.

In humans it is quite often one male-one female relationship, so you have to bump number of cells in ejaculate so that mating season is fruitful.

Final point of real-life statistics:

Of the 300 million sperm ejaculated, only about 200 or so will survive to reach the egg cell and only one will succeed in fertilizing it.

You need to beat these odds ($10^{-6}$) in order to reproduce. Why female reproductive organs are so restrictive? Probably because there is some link between male's overall fitness and number of sperm cells. Fit males (those that hunt better than others) are able to spend more energy for generating sperm, rather than "feeding" their own body. If they able to "feed" 300 million sperm cells, they probably will be able to feed a child.

$\endgroup$
  • 1
    $\begingroup$ Your theory seems to go straight against the sperm competition theory? $\endgroup$ – Alex Jul 24 '15 at 11:10
  • $\begingroup$ I'm a bit suspicious of the implied statistics here... If there were 300 million sperm, and 40 million reached the egg, still only 1 would fertilise it (except in rare cases). Also you're relying on something like bodily system integrity to imply sperm count reflects general fitness. I'm not sure if something like this has been demonstrated. I'd be very interested if you could provide a citation for that line. $\endgroup$ – James Apr 10 '16 at 4:30

protected by Community Apr 10 '16 at 6:10

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.