A barefooted runner takes to the streets. Charlie Riedel/AP hide caption

itoggle caption Charlie Riedel/AP

Researchers dyed sperm from each deer mouse a different color; in this case, red represents one male and green another. The sperm joined together with their own kind, as seen here, allowing them to swim faster than those that did not cluster together. Heidi Fisher hide caption

itoggle caption Heidi Fisher