| https://en.wikipedia.org/wiki/Polyspermy has some short but interesting discussion of the theoretical background. A polyploid zygote is completely nonviable, a pure waste of the egg, sperm, and other mating effort that went into it. It's in the interest of the mother and the father for the egg to be fertilized by exactly one sperm. But it's in the interest of an individual sperm to be the one that fertilizes the egg instead of dying uselessly. So they get faster, more efficient, and more penetrant over time. When they're too effective, multiple sperm may get into an egg before the defenses are up. (Which, again, just kills the sperm that "won" the race, so you don't expect to see incredibly rapid progress in this area.) Oceangoing eggs respond by having a fertilized egg's defenses go up really quickly, because the egg is floating around in the ocean and it's hard for the mother to influence that environment. Female mammals have responded with slower defenses, but internal characteristics that tend to retard or hurt sperm as they make their way towards the egg, meaning that it's rare for multiple sperm to all get there at once. This is good for avoiding polyploidy, but bad for fertility -- it is not necessarily the case that even one sperm will make it. It's interesting to see competition among sperm hurting the reproductive chances of the male and female producing and accepting it. |