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 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.