Women's reproductive organs are designed to reject weaker sperm, and only the fastest candidates through a series of "gates," researchers at Cornell University have found.
They noted that weaker sperm caught themselves in currents at the gates – or "strictures" – while the fastest and strongest swimmers could withstand the oncoming current and crowd through the opening.
"Strictures in the seminal swimming channel play a goal-like role," the authors said. "Sperm at higher speeds than a threshold can pass the constriction, while slower sperm accumulates than the threshold below the constriction."
The sperm "forms a hierarchy" as it accumulates on the strictures and waits to get through, the team added, floating on a butterfly-shaped path as they move.
The strongest swimmers get closer to the stricture's mouth, while the slower cells stay behind.
"This hierarchical structure compels competition among the sperm, with the most intense competition between highly mobile micro-swimmers compared to the slower sperm," they wrote.
The team used both human and bull sperm in the tests, both of which gave similar results.
Their findings, while not unexpected, shed further light on which sections of the sperm travel are the most difficult to navigate and which causes so many to fall by the wayside.
The "gate-like behavior of the stricture indicates a motility-based selection mechanism that may be used by the female reproductive tract," they said. This means that the most moving cells – which can move best – are the most likely to go fast.