Image: AP

Fruit flies have enormous sperm. This is a well known fact in the scientific community—so well known, in fact, that there’s a name for it: the big sperm paradox. But the massive, spermy problem has long confounded scientists, who couldn’t figure out why such a tiny creature needed such humongous baby batter soldiers. Until now.

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A new paper published today in Nature, however, has introduced a mighty intriguing explanation. Researchers found a link between the size of the semen receptacle in female fruit flies and the length of sperm in males, which suggests that it’s the ladies who are actually calling the evolutionary shots.

“Specifically, longer sperm are superior at displacing, and resisting displacement by shorter competitor sperm within the semen receptacle, and longer semen receptacles drive sperm-length evolution by enhancing this competitive advantage,” the paper reported.

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Or, as study author Scot Pitnick told the Associated Press, “It’s the females that are driving the evolution of these absolutely absurd, ridiculous traits in males.” (Pitnick, it is worth noting, has a 50-inch sperm tattoo around his arm. Now that’s dedication!)

When you hear the phrase “giant sperm,” you might be thinking of sperm that is a little bit larger than normal. You would be incorrect, because fruit fly sperm is roughly 2.3 inches long. That’s about 20 times longer than fruit flies themselves, and about a thousand times longer than human sperm. It’s not freely hanging around, however—it’s smushed up into something the AP compared to “a ball of yarn.” Sperm yarn forever!

Sperm yarn! (Image: Nature)

Pitnick and his fellow study authors compared fruit fly sperm to other sexually selected traits, like antlers and pheasant tails, but concluded that the sperm was “possibly the most extreme ornaments in all of nature.” In other words, next time you (or your partner) brings up your or his impressive sex skills, just remember this: it can’t beat a fruit fly’s.

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[Nature via Popular Science]