Who woulda thunk it?

File this one under "Learn something new every day" ... it's from an e-book I read recently entitled "Harry the Hunter and Gida the Gatherer" by relationships author and sociologist Kara Oh.

"In recent research it has been discovered that there are several kinds of sperm. There are 'odds and ends' which are designed explicitly for the sole purpose of taking up space and adding to the overall mass, because a larger mass more successfully covers the opening to the cervix, thus raising the odds of impregnation as well as impeding the likelihood of another man impregnating the woman.
Then there are 'warrior' sperm that attack any other man's sperm that might be in the woman, and 'gatekeeper' sperm that hover around the egg, allowing only "brother" sperm to impregnate the egg.
If a man thinks there's a possibility that another man has had sex with his partner, his body makes more of the 'warrior' sperm. This is competition at the most primal level."