Sperm have to fight when a female has multiple partners — which is pretty normal behavior in nearly all animals. Syracuse University researchers actually filmed the little sperm in action. By using fluorescent tagging technology, the scientists could watch the sperm compete in the female reproductive tract.

The Scientist reports:

"The paper is really the methodological breakthrough that enables us to capture a glimpse of sperm competition in action," said evolutionary biologist Tommaso Pizzari of the University of Oxford, also not a co-author. "What happens inside the female has remained, up till now, a bit of a black box."

Watch Drosophila sperm fight. Note: One male's ejaculate is colored green and the other one is tagged in red.

The study's author, evolutionary biologist Scott Pitnick at Syracuse University told The Scientist: "We were baffled at first by the incredible activity of sperm."

So far, the scientists have seen two species of Drosophila and a flour beetle. Next up for examination are ants and bees — previous studies have shown that in these species, semen has a memory for its sperm and knows when the sperm is not its own. Understanding how sperm compete will unlock clues on paternal variation.

This post was originally published on Smartplanet.com