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."


The study, published March 18 in Science, sheds new light on postcopulatory sexual selection—a "powerful force for evolutionary change," according to Pitnick. When a female mates with more than one male (a common practice in fruit flies and other species), postcopulatory sexual selection is what drives paternity. The researchers conclude that the timing of sperm release from female storage organs, the distribution of competing sperm, "fair raffle" sperm use (meaning no sperm favoritism from the female or the environment) and the "surprising level of sperm mobility" all influence postcopulatory sexual selection.

So who wins? Although every sperm moves through female storage organs at about the same pace, the sperm from the most recent mate tends to oust those from earlier mates. The study corroborates previous reports that so-called "second-male" sperm can displace "first-male" sperm, but does not suggest that sperm can incapacitate their competitors (in contrast to earlier reports of "killer" or "kamikaze" sperm).

The ability to watch previously unobservable events occurring between insemination and fertilization might have huge implications for the fields of reproductive biology, sexual selection and speciation. "I suspect we have just scratched the surface of using this material," Pitnick says.

Top image of the reproductive tract (less ovaries) of female D. melanogaster initially mated to GFP-sperm male then remated to RFP-sperm male. Green sperm heads have left the storage organs and can be seen mixing with red sperm heads in the bursa. Bottom image of the seminal receptacle (SR) and one spermatheca of a female D. melanogaster initially mated to GFP-sperm male then remated to RFP-sperm male. Sperm are mixed in the SR, yet only green sperm reside in the spermatheca. Images © Science/AAAS