• When sperm is observed under a microscope, 2D lenses create the wrong impression about the way they move
• The tail doesn’t move from side to side, but only wiggles on one side
• Sperm actually roll as they swim, allowing their one-sided stroke to propel them forwards  

If you ever had a sex-ed class in school, you have probably seen a visual of sperm swimming with a wagging tail. Now, high-tech tools have shattered that view of how sperm move.

More than 300 years ago, a Dutch scientist used an early microscope to observe human sperm in motion. He saw that they appeared to swim using a tail that moved from one side to the other.

But scientist Antonie van Leeuwenhoek only had access to a 2D microscope.

And sperms’ rapid spinning causes an illusion when seen from above with a 2D microscope, according to a study published on 31 July in the journal Science Advances. That illusion makes it appear that the tail moves from side to side, as Leeuwenhoek described.

Similar to an otter

Now, an international team has deployed a high-speed camera, high-tech microscope and mathematics to show that the sperm tail only wiggles on one side.

A one-sided stroke would typically cause something to swim in circles, but the researchers explained that a sperm’s ability to move forward is similar to an otter.

“Human sperm figured out if they roll as they swim, much like playful otters corkscrewing through water, their one-sided stroke would average itself out, and they would swim forwards,” said researcher Hermes Gadelha. He’s a senior lecturer in the department of engineering mathematics at the University of Bristol in the United Kingdom.

To this day, when sperm is looked at under a microscope in clinics or laboratories, 2D lenses are typically used, promoting the illusion first seen by van Leeuwenhoek centuries ago.

The study authors hope that their findings will improve understanding of the mysteries of human fertility and reproduction.

“With over half of infertility caused by male factors, understanding the human sperm tail is fundamental to developing future diagnostic tools to identify unhealthy sperm,” Gadelha said in a university news release.

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