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The sperm of some species of fruit fly are up to 5.8 centimetres long — around 20 times as long as the fly itself (pictured, two Drosophila bifurca sperm spill out of the male's ruptured seminal vesicle). Giant sperm tails are energetically expensive to produce, and their evolution requires intensive sexual selection. And yet theory predicts that sexual selection should steadily weaken as sperm get larger and their numbers decline. On page 535, Lüpold et al. investigate this 'big-sperm paradox' (S. Lüpold et al. Nature 533, 535–538; 2016).

But what is the benefit to females? Only large, healthy males that carry 'good' genes can produce long sperm in sufficient quantities to outcompete other suitors. Therefore, a long seminal receptacle maximizes a female's chances of producing offspring that have high-quality genes. Both sexes win in this evolutionary arms race.

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