Sperm do lopsided barrel-rolls. They don’t have propeller tails.

Science News dashes the popular image of sperm as swimming furiously by spinning their tails like boat propellers. Instead, the little guys only move their tails in one direction, and keep rolling their heads and bodies around to compensate, like tiny corkscrews:

Over 300 years ago, microscopy pioneer Antonie van Leeuwenhoek described sperm tails swaying in a symmetric pattern, like “that of a snake or an eel.” The prevailing view that sperm tails move in a balanced way, however, doesn’t capture what actually happens in three dimensions, researchers report July 31 in Science Advances.

High-speed 3-D microscopy of human sperm swimming freely in the lab revealed that the cells corkscrew as they move, consistent with previous studies. The sperm almost seemed to be drilling into the surrounding fluid, says Hermes Gadêlha, a mathematician at the University of Bristol in England.

Using automated tracking of swimming sperm and mathematical analyses of position data, Gadêlha and colleagues broke sperm tail movement down into two components. Surprisingly, one was a wiggle to only one side of the cell. It’s like someone swimming using just one side of the body, Gadêlha says. By itself, such a lopsided stroke would lead to swimming in circles.

But a second component of tail movement causes the sperm to rotate, balancing out the lopsided strokes. From above, the sperm tail looks like it is beating symmetrically, as has been described historically. But a more complex, 3-D movement keeps the sperm swimming straight ahead.