Female fruit flies use a nutritional sensor in their brains to chemically inspect a male's ejaculate before deciding whether to keep his sperm.

Mating is often viewed as a simple exchange of genetic material, but it is also a transfer of valuable nutrients. Male fruit flies include a specific sugar called a galactoside in their ejaculate as a nuptial gift for the female. Dedicated neurons in the female's brain detect this chemical signal to assess the male's health and quality. If the signal is weak, the female may choose not to store the sperm or invest in the resulting offspring. This hidden quality-control mechanism proves that females are making high-speed chemical calculations about their partners during the act of mating.