Sperm death and dumping in Drosophila

Mating with more than one male is the norm for females of many species. In addition to generating competition between the ejaculates of different males1,2, multiple mating may allow females to bias sperm use3,4. In Drosophila melanogaster, the last male to inseminate a female sires approximately 80% of subsequent progeny5. Both sperm displacement, where resident sperm are removed from storage by the incoming ejaculate of the copulating male6, and sperm incapacitation, where incoming seminal fluids supposedly interfere with resident sperm7, have been implicated in this pattern of sperm use5,6,7,8,9,10,11,12. But the idea of incapacitation is problematic because there are no known mechanisms by which an individual could damage rival sperm and not their own. Females also influence the process of sperm use13,14, but exactly how is unclear. Here we show that seminal fluids do not kill rival sperm and that any ‘incapacitation’ is probably due to sperm ageing during sperm storage. We also show that females release stored sperm from the reproductive tract (sperm dumping) after copulation with a second male and that this requires neither incoming sperm nor seminal fluids. Instead, males may cause stored sperm to be dumped or females may differentially eject sperm from the previous mating.