Scientists call it the “big sperm paradox.” Fruit flies (Drosophila) have evolved colossal sperm cells that can reach lengths up to 5.8 centimeters, roughly 20 times the length of its body and about 1000 times longer than a human’s. But why would the insects invest so much energy in making giant sperm when—especially in a species like this, where one female is inseminated by many mates—evolution drives males to produce more sperm cells, not bigger ones. To find out, researchers bred fruit flies under different conditions and amounts of stress to create males with varying levels of fitness and then analyzed the males and their sperm with genetic tests. Compared to their larger, fitter counterparts, smaller, less-fit males needed to invest more resources to produce large numbers of big sperm, the team reports online today in Nature. This allows only the fittest males to mate with many partners and thus increases the female’s likelihood of receiving sperm from high-quality mates. And it does appear that the females are the ones driving this process: They’re evolving longer seminal receptacles (seen above coiling on top of itself under high magnification) to stiffen the competition and select for only the longest sperm from the healthiest mates. In a way, the large sperm of fruit flies are a bit like the peacock’s giant tail or the stag’s antlers—a sign that this male is the one you want to be with.