Once upon a time, people would not believe that when a horse ran, all four of its legs were off the ground at the same time. It took high-speed photography to prove that a running horse actually does go airborne. Now high-speed photography combined with state-of-the-art microscopy has revealed that the well-known image of a swimming sperm, with tail beating rhythmically back and forth, is actually an illusion.

The sperm's tail only wiggles in one direction. While this should have them moving in a circle, sperm have found a way to roll with this handicap and move forward instead.

It took a 3D microscope paired with a high-speed camera to observe sperm freely swimming in three dimensions.

“Human sperm figured out if they roll as they swim, much like playful otters corkscrewing through water, their one-sided stoke would average itself out, and they would swim forwards,” said study co-author, Hermes Gadelha, head of the Polymaths Laboratory at the University of Bristol’s Department of Engineering Mathematics and an expert in the mathematics of fertility.

The corkscrew-like motion isn't visible with a conventional two-dimensional microscope. It took a 3D microscope paired with a high-speed camera to observe sperm freely swimming in three dimensions.

Fertility clinics and research labs, even high-tech ones, still use 2D technology to observe sperm movement. Despite their computer-assisted semen analysis systems, there may be many more details they are missing that could show up if they used a 3D system, as has happened with mammograms.

Fertility rates haves been on the decline in the industrialized world in recent years, and and some studies point to sperm quality as a major factor. Perhaps a closer look at them swimming in 3D will lead to further diagnostic tools and eventually treatments for low-quality sperm.

Over half of infertility is caused by male factors, according to the researchers. While the exact cause can't always be pinpointed, a more detailed examination of their sperm couldn't hurt.

The use of 3D microscopy to study sperm movement was pioneered by researchers from the Universidad Nacional Autonoma de Mexico who believe the discovery will advance not just our understanding of sperm motility, but also its impact on natural fertilization. “So little is known about the intricate environment inside the female reproductive tract and how sperm swimming impinge on fertilization. These new tools open our eyes to the amazing capabilities sperm have,” said co-author, Alberto Darszon, of that university's Department of Developmental Genetics and Molecular Physiology.

An article on the study appears in Science Advances.