Sperm is highly competitive.
From Science Daily: When it comes to mating and determining whose sperm reaches the elusive egg, females control both the playing field and the rules of the game .
Even the length of sperm makes a difference. “Females choose among males based on the length of their sperm. Long sperm tails are thus the post-copulatory, cellular equivalent of long peacock tail feathers.”
Research also shows that speed of sperm matters too: see Primate sperm competition: speed matters.
The evolutionary implications of sperm competition were first recognized in 1970, when it was realized that competition between males could continue beyond copulation and insemination, and that what males really compete for is fertilizations rather than mates.
Sperm competition generates conflicting selective pressures on males. On the one hand, selection will favor males that manage to fertilize the ova of already-inseminated females, but on the other, it will also favor males that prevent other males from fertilizing the ova of females they themselves have recently inseminated.
Initially, it was assumed that females had little to gain from copulating with more than one male, and it was also assumed that they were either passive or reluctant participants in sperm competition.
More recently, it has been recognized that not only do females actively seek multiple partners, but that they do so because they may obtain evolutionary benefits from having their ova fertilized by particular males. As a result, females of some species may have evolved the ability to discriminate in their reproductive tract between the sperm of different males (referred to as cryptic female choice).
Sperm competition may therefore result in the rapid coevolution of male and female reproductive traits.
Adaptations to Sperm Competition
All parts of a male’s reproductive anatomy—the testes, sperm storage sites, the penis, accessory glands and the seminal fluids they produce—and the sperm themselves have been shaped by sperm competition (and cryptic female choice).
*Eggs are relatively few, large, and do not move, whereas sperm are many, small, and mobile.