Sperm competition is a form of post-copulatory sexual selection  whereby male ejaculates simultaneously physically compete to fertilize a single ovum. Physiological evidence, including testis size relative to body weight and the volume of sperm in ejaculations, suggests that humans have experienced a low-to-intermediate level of selection pressure for sperm competition in our evolutionary history. Evidence suggests that, among the great apes, relative testis size is associated with the breeding system of each primate species. The volume of sperm in ejaculates scales proportionately with testis size and, consistent with the intermediate weight of human males testis, ejaculate volume is also intermediate between primates with high and low levels of sperm competition.
Sperm competition , a special form of mating competition that occurs in sexual species when females accept multiple mating partners over a relatively short period of time. The potential for overlap between the sperm of different males within the female has resulted in a diversity of behavioral adaptations and bizarre strategies for maximizing paternity. Sperm competition is thought to be the primary reason why males offer nuptial gifts such as food to females or allow females to cannibalize them. Such nuptial gifts are best thought of as mating effort that is, effort directed at increasing the number of offspring a male sires , because they are usually not available at the time of birth or hatching to benefit the offspring sired by the male presenting the gift.
Choosy Eggs May Pick Sperm for Their Genes, Defying Mendel’s Law
Still others lack the energy to finish the long journey through the female reproductive tract, or they get snared in sticky fluid meant to impede all but the strongest swimmers. For the subset of a subset of spermatozoa that reach their trophy, the final winner would be determined by one last sprint to the end. The exact identity of the sperm was random, and the egg waited passively until the Michael Phelps of gametes finally arrived.