Sperm competition occurs when a female mates with more than one male before fertilization occurs. The female has the sperm of more than one male inside her body, so an element of competition exists between the sperm. Each set of sperm travels toward a female’s egg with the objective of fertilizing it, and each male typically wants his sperm to reach the egg first. The behavior of the sperm themselves does not change during sperm competition because they cannot know that they are in competition. The competition between sperm from the same male is not considered to be sperm competition in this sense.
The sperm of both males — or all of them, if there are more than two males involved — are in competition to reach the female’s egg first. Therefore, only the fittest and most physically able of the sperm will succeed. To help sperm do better in the competition to fertilize females, males have evolved mechanisms to increase their chances of success. Generally, the greater the competition between males, or the more promiscuous the female animal, the more that evolution has needed to play a role in equipping the male and his sperm for the task of effective fertilization.
Sperm quality usually is the deciding factor for a male’s sperm to win the sperm competition and fertilize the female’s egg. There are several factors that make a male's sperm superior to other males sperm. Among these qualities are longevity, speed and energy.
Evolutional changes have occurred in which sperm are produced to the optimal size to be able to swim as fast as possible, given the amount of energy required to reach the female’s egg. Males have also evolved to produce sperm in greater numbers where there is greater sperm competition. Throughout the animal kingdom, large testes are observed where this is so.
As a result of sperm competition, males will guard a female from other males before and after mating. Some fish, such as guppies, attempt to mate with a female directly after she has given birth, to ensure that he mates with her first. Males from species of birds, reptiles and mammals will all fight for the right to mate with a female.
Colorful shows are put on for females where sperm competition is great. One example is the colorful peacock who displays his fan of feathers to impress a female more than the rival male. Some animals, such as spiders, insert a copulatory plug into the female after mating, to prevent another male from attempting to mate.
Some females will mate with multiple males in a short time. The black-winged damselfly is one such animal. The male of the species has a brush-like penis that he uses to clean the female of the sperm already deposited inside her.