Sperm kill rival human
In the battle of the sexes, there are the formal rules -- the rich variety of variously heeded laws and customs by which each society tries to constrain reproductive behavior. Then there are the other rules, often in conflict with the first, which govern love and desire and the delicate games men and women play with one another. If these other rules, the ones built into human nature, could be specified, they would not tell what is right -- that is the province of the first set of rules -- but they would help understanding of how human sexual and social behavior have evolved. Primatologists, who study the group of mammals that includes monkeys, apes and people, have developed some surprising ideas about the nature of the human mating system, suggesting that women may not be as faithful as their mates may suppose.
Sperm Warfare (Or: Why it Takes 1 Billion Sperm to Make One Zygote)
Sperm competition - Wikipedia
In the closing paragraph of on the origin of species Darwin famously said that nature was a war in which individuals struggle against each other and the environment for survival. However, while survival may be important from an individuals point of view, from an evolutionary perspective mere survival is not enough. Of course, survival is important too, but only when it leads to reproduction. In most species the reproductive success of females is limited by the rate at which they can produce offspring. When a female is pregnant or carrying eggs she has no choice but to wait until she has given birth or laid her eggs before she can reproduce again, and this can take a long time. Males have no such constraints to their reproductive success and can potentially mate with hundreds of females over their lifetime and raise an enormous number of offspring.
Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg  during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase males' chance of reproductive success. Sperm competition is often compared to having tickets in a raffle ; a male has a better chance of winning i.
We all know that individuals fight over potential love interests. But you might be surprised to hear that the fierce rivalry continues behind the scenes — in the form of sperm competition. This is when the sperm of two or more males compete inside the reproductive tract of a female, to fertilize the eggs, something that is widespread in the animal kingdom.