It ain’t easy being a sperm. Each load of semen in humans contains between 60 million to 100 million sperm – but there’s only one mature egg per cycle. To weed out the weak sperm and source only the best-of-the-best, the female reproductive tract is essentially an assault course, rigged with obstacles and booby traps. Ultimately, there can be only one winner. 

Reporting in the journal Science Advances, scientists at Cornell University in New York have detailed how sperm navigate the structure of the female reproductive tract. They discovered a series of microchannels and tight spots (also called strictures) inside the female reproductive tract that were developed to encourage fierce competition between sperm, ensuring only the most nimble and able make it to the egg.

"... strictures inside the sperm swimming channel play a gate-like role," wrote the authors of the study. "That is, sperm with velocities higher than a threshold value can pass through the stricture, whereas sperm slower than the threshold accumulate below the stricture."

The researchers used computer simulations and bull sperm (which have a similar method of locomotion to human sperm) to observe how sperm travels from the cervix to the egg. Based on these observations, they created an artificial bottleneck to mimic the tight channels of the reproductive tract and then tracked the movement of the sperm. They noted that the slower sperm tended to gather near the “gates”, unable to slip through the narrow channels, and were swept back by the surrounding flow. Fast sperm, however, managed to nip through the channel with ease.

“This gate-like behavior of the stricture suggests a motility-based selection mechanism that may potentially be used by the female reproductive tract to select for sperm with the highest motility,” the researchers concluded.

Low sperm motility does raise many issues with fertility, but there’s no evidence that faster swimming sperm offers any genetic advantages over their slower swimmers. However, these new insights could potentially be used to assist couples who are having trouble conceiving, especially if low sperm motility is an issue. Dr Harry Fisch, a clinical professor of urology and reproductive medicine at Weill Cornell Medical College, who was not directly involved in the study, told HealthDay that the findings could be used to develop treatments that compensate for the sluggish swimmers, such as injecting the sperm farther up into the female reproductive tract to give their partner's sperm a "leg up", so to speak.