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(March 19) — They’re speedy, agile and extremely competitive. Male sperm, that is. That’s the conclusion of researchers who were able to watch the contentious battle for fertilization firsthand after they created the first glow-in-the-dark sperm.

A team of scientists at Syracuse University, whose work is published in this week’s issue of Science, used genetic manipulation to produce fruit flies whose sperm heads glowed either red or green. By tracking the fluorescent sperm as it traveled through the inside of a female fruit fly, they were able to observe reproductive events from an entirely new perspective.

It’s a step that will transform what scientists know about reproduction, and potentially how doctors treat human infertility.

Scientists used genetic manipulation to produce fruit flies whose sperm heads glowed either red or green in order to observe what happens as the sperm make their way through the female reproductive tract.

One of the pillars of evolutionary science has long been the behavior of different male sperm. When a female mates with several partners, the sperm that successfully fertilizes her egg will determine which traits carry on to the next generation. In this case, female fruit flies were mated with two partners: one with green sperm, the other with red.

The Syracuse scientists are the first to watch the race for fertilization as it happens. Lead researcher Scott Pitnick said the sperm “exhibit highly complex behavior” as they maneuver through the female reproductive tract.

Despite decades of reproductive research, often using fruit fly specimens, scientists remain largely mystified about what goes on between insemination and fertilization. Observing glow-in-the-dark sperm will give researchers the chance to examine every maneuver and reproductive milestone.

And tracking the fate of every single sperm could have implications that extend beyond better knowledge of evolutionary selection in small organisms. The team has already created glow-in-the-dark sperm samples of other animals.

“It’s seeing all the novel observations, the complexity of what sperm do inside the female reproductive tract that no-one has ever been able to observe before,” Pitnick told the BBC. “That’s really knocked us out.”