A new Nature Communications study finds that human sperm swim faster and straighter near surfaces than in open environments, which may reflect an evolutionary strategy to adapt to narrow reproductive systems. The sperm of most organisms possess a long tail, the flagellum. Sperm move forward by spirally flapping their flagella. In aquatic animals, such a swimming method allows sperm to cross the water and find their home, the egg. However, for those animals whose reproductive systems are entirely inside their bodies, they sperm need to contend with a large surface area to do so. To examine this closely, a team of foreign researchers photographed human sperm moving one micron from a glass surface and compared it to human sperm swimming in a large amount of free solution. When sperm swim on a surface, they adopt a special gliding swimming style that allows them to swim faster and straighter along the surface. This gliding swimming pattern occurs more frequently when sperm are moving in mucus that mimics the human female reproductive tract. In contrast bull sperm swim more slowly when they are tightly attached to a surface. The oviduct, where human fertilization occurs, is a closed environment with a large amount of mucus. The oviduct in cattle is much larger than in humans, and bull sperm need to interrelate with the surface much less frequently. Thus, the gliding swimming pattern adopted by human sperm appears to be an evolutionary pattern appropriate for the narrow reproductive system of the human body.