Sperm competition is thought to be responsible for the tremendous inter- and intraspecific variation in sperm number1 and size2. But why several animals produce a range of sperm types3,4, some of which are incapable of fertilizing the female's eggs4, has remained unexplained for nearly 100 years5. We have found that non-fertile sperm protect a male's reproductive investment by delaying female remating in the polyandrous green-veined white butterfly, Pieris napi (Pieridae).