We know that the upshot of sex between a man and a woman is more than 50 million sperm racing to reach one egg, though exactly how sperm swim through a woman’s body and how just one survives the journey to fertilize an egg is full of mystery.

However, a researcher at Kyoto University says he has found a major clue: a simple mathematical formula to explain the rhythmical swim of a sperm.

Kenta Ishimoto, an assistant professor of applied mathematics at the university, together with researchers at the universities of York, Birmingham and Oxford, has found that the sperm’s tail creates a “four-beat” rhythm that pushes the sperm forward but also pulls the head backward and sideways.

The findings could provide insight into male infertility, by mathematically explaining the characteristics of active sperm, he said.

“Until now, it has been said that the tail of a sperm simply moves forward by pushing fluid around it,” Ishimoto said by phone. “But we have found the mathematical patterns of the moves by calculating the flow of fluid around it. It showed a clear, four-beat step, something that we had never imagined was happening.”

The research was published Friday in the U.S. journal Physical Review Letters.

While the research focused on the movement of one sperm, Ishimoto wants to apply the mathematical model to predict how larger groups of sperm behave and interact, as they travel through the narrow, maze-like fallopian tubes.

Ishimoto, who has studied sperm movement since 2013, says his research stems from his genuine interest in the wonders of life and its origin.

“Sperm swim very differently from us, and it’s often said that they look as if they are swimming in a thick fluid like honey,” he said. “I’ve wondered how it feels to be a sperm swimming like that. It’s like finding out an episode of my life, the journey I took before I was born.”

While fertility or infertility cannot be explained purely by dynamics, as it is also influenced by the DNA of sperm, Ishimoto said he wants to further his research by collaborating with experts in other fields.

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