Female fruit flies can feel when a sexual partner is a good fit. Scientists have long known that proteins in a male fly’s ejaculate make female flies temporarily lose interest in other partners. It’s a trick male flies use to raise the chance that eggs get fertilized with their sperm, not someone else’s. But a new study suggests that the sensation of sex—regardless of sperm—can also make females reject other partners, researchers report May 6, 2019, in the journal Neuron.