The reproductive tract of the female is craftily crafted to clean the poor sperm.
Scientists at Cornell University of New York have come to this conclusion, who say a number of 'points' - such as narrowing between the uterus and the fallopian tubes.
As a result, only the strongest sperm cells can walk their way through the cramping to reach the egg, creating a quality control process for fertilization.
Finding can help improve fertility manifestations and improve a couple's chances of having children.
"If you look at the anatomy of the reproductive system in mammals, you can see that the canal dimensions leading to the eggs are not constant," said Dr. Alireza Abbaspourrad, chemist and lead author of the study.
Dr. Abbaspourrad and his team used human sperm and bull for comparison, then analyzed their movement into a "microfluidic" device that imitates a woman's uterus.
They found that the sperm assembles at this point and only the fastest sperm passes.
They also noted that at this point, the sports sperm competes only with others of similar quality - not slower or weaker.
