(CNN) - The race to fertilize an egg is a ruthless one -- and now, scientists have shed more light on the punishing obstacle course attempted by hundreds of millions of sperm cells in their bid to get there first.
Women's reproductive tracts are shaped in a way that rejects weaker sperm, allowing only the fastest candidates through a series of "gates," researchers at Cornell University have found.
The team, writing in the journal Science Advances, used a number of models and computer simulations to observe sperm along their journey from the cervix to the egg.
They noticed weaker sperm got caught in currents at the gates -- or "strictures" -- while the fastest and strongest swimmers were able to withstand the oncoming flow and force their way through the opening.
"Strictures inside the sperm swimming channel play a gate-like role," the authors explained. "Sperm with velocities higher than a threshold value can pass through the stricture, whereas sperm slower than the threshold accumulate below the stricture."
Sperm "form a hierarchy" while accumulating at the strictures and waiting to get through, the team added, swimming in a butterfly-shaped path as they move.
During that process, the strongest swimmers get closer to the stricture's mouth, while the slower cells are left behind.
"This hierarchical structure imposes competition among the sperm, with the fiercest competition occurring among highly motile microswimmers in comparison with the slower sperm," they wrote.
The team used both human and bull sperm in the tests, with both providing similar results.
Their findings, while not unexpected, shed further light on which sections of the sperm cells' journey are most difficult to navigate, and what causes so many to fall by the wayside.
The "gate-like behavior of the stricture suggests a motility-based selection mechanism that may potentially be used by the female reproductive tract," they explained, meaning the more motile cells -- those most able to move independently -- are the most likely to pass through quickly.
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