In situations where a female copulates with several males in quick succession, only the best sperm, marked by speed, size and viscosity, wins the race to fertilise the egg, says a study.
In nature, it is not unusual for a female to copulate with several males in quick succession - chimpanzees are an example.
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"The sperm of the different males then compete within the female to fertilise the eggs," said study author Steven Ramm from the Bielefeld University in Germany.
The researchers showed that it is not just the amount of sperm that is important, but also its form.
"Generally speaking, the best sperm wins. This may involve its speed or also be due to the amount of sperm transferred. It can also be useful for the seminal fluid to be viscous, meaning it sticks inside the female reproductive tract to try to keep other sperm at bay," Ramm added.
"Even just the size of the individual sperm cell may provide a competitive advantage," Ramm said.
"Producing the optimum sperm type occurs in the testis, during spermatogenesis," he added.
Sperms are considered to be the most complex type of cell in the entire animal kingdom. The findings appeared in the journal Molecular Human Reproduction.