Which sperm will win the race to the egg: the green one or the red one?

Once they're inside the female reproductive organs, sperm pull out all the stops to outrace their rivals to the egg—especially if the opponent comes from another male. The process that determines which sperm wins, called "postcopulatory sexual selection," has been difficult to tease out, until now.

By genetically engineering fruit flies that express green or red fluorescent (glow-in-the-dark) proteins in their sperm heads, researchers from Syracuse University have made the race to the egg spectator-friendly. These glowing sperm can be detected through the female fruit flies' reproductive organs, allowing the fierce competition to be watched in real time through a special microscope. "Our jaws hit the floor the first time we looked through a microscope and saw these glowing sperm," lead author Scott Pitnick said in a prepared statement. "It turns out that they are constantly on the move within the female's specialized sperm-storage organs and exhibit surprisingly complex behavior."