All sperm perform the same basic job: They fertilize egg cells. But in a new study, researchers have figured out that size matters, and it’s largely the female that pushes sperm to be big or small.

Sperm cells come in a huge variety of sizes. For instance, the parasitoid wasp Cotesia congregata produces little swimmers that are less than one-thousandth of a centimeter long, while fruit flies make sperm with 2.3-inch (6 cm) tails that coil tightly to fit inside their tiny bodies. 

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