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Watch a 3D video reveal how sperm really swim

*Editor's note, 20 May 2021, 12:45 p.m.: The study described in this article has been retracted. The retraction notice states: “After publication, readers identified that although our 3D flagellar experimental data and analyses are sound, the conclusion of flagellar asymmetry and anisotropy cannot be drawn unequivocally using only 3D flagellar waveform data. Because of this, the description of asymmetry and anisotropy in the article is currently incomplete.”

Watch a human sperm under a microscope, and it will appear to swim like an eel wiggling its tail through the water. But a new study reveals that sperm actually swim in a much more chaotic manner—one they’ve been able to harness for maximal speed.

Researchers scanned human sperm samples with a 3D microscope and a high-speed camera—one that could keep up with the speedy swimmers, which can manage 20 to 30 swimming strokes per second.

Though in two dimensions sperm appear to lash their tail from side-to-side in symmetric strokes, the reproductive cells actually move in a lop-sided manner, beating their tails to one side only (as seen in the video above), the team reports today in Science Advances.

This movement should cause the sperm to swim around in circles, but they have found a solution that allows them to propel forward, the team found. Their heads spin in one direction, while the tail rotates around it in the opposite direction in a process known as precession—much like Earth spinning on its axis while orbiting the Sun. This corkscrewing motion balances out in an overall forward trajectory, creating the illusion of symmetry from asymmetry.

Subtle changes in sperm movement could affect fertility, the team notes, so these findings could pave the way toward new fertility treatments or even a male contraceptive that could target sperm mobility.