To get a better look, researchers from the University of Toronto used total internal reflection fluorescence (TIRF) microscopy, a technique that allows them to look at the 3D movements of particles in fluid within 100 nanometers of a flat surface. The researchers compared the movements of human and bull sperm in liquids of varying thickness, or viscosity. They found that sperm from both species would intermittently slither like a snake, moving their “tails” back and forth on a two-dimensional plane—the first time such a movement has been observed in sperm. When the liquid was more viscous, the human sperm moved this way more often, and moved 50 percent faster than the bull sperm. The researchers think that the human sperm might have to slither more often because human reproductive tracts are smaller and more confined than those of cows.