As I understand it, that’s what kills off a lot of the sperm. Or is it beneficial bacteria? Anyway, that’s why there are so many sperm in an ejaculation.
The idea is that the healthiest sperm make it all the way to the egg.
This is, indeed, fun. So many great answers!
The journey to fertilization is like running a gauntlet.
First of all, there’s not much food in a sperm. They’re little, without the relatively big store of carbohydrates that the fat ol’ egg has. So there’s not a lot of power in the battery. When it runs down, the sperm is dead.
Secondly, the intrauterine (and vaginal) environment is generally hostile to sperm. The pH isn’t optimal, because pH is a good way to foil bacteria. The same pH that kills bacteria by melting their cell membrane also tends to do the same to sperm. (Can you tell I’m being a little vague here? That’s cause I don’t remember if it’s too acidic or too basic for healthy sperm and I’m too lazy to go look it up.)
Thirdly, the fallopian tubes (where fertilization occurs) are hard to get up into. There’s not a nice slick river of moisture that the sperm can use to swim in. In fact, there’s some liquid there with elements that actually act as a net, or sieve, and trap sperm so they can’t get inside very well.
What’s the solution? Cervical mucus! It changes for a few days before ovulation, becoming slick and very wet, providing a slip and slide for the sperm. They can swim much faster, using less energy, if there’s good fertile quality cervical mucus present. Cervical mucus also changes the pH, making it more hospitable to sperm. It may (I don’t think this has been proven, but it’s readily hypothesized) also provide an extra nutrient source for the sperm.
Cervical mucus is really cool! My enthusiasm for cervical mucus cannot be overstated. Maybe that’s why I’m not invited to many parties.
Great answer!! I think it’s pretty cool too.