Drosophila melanogaster males increase the number of sperm in their ejaculate when perceiving rival males.

It is common for females from many species to mate with multiple males within one reproductive cycle. As a result, sperm from different males come into contact in the female reproductive organs, where they compete for ova fertilization. This sperm competition appears to drive the ejaculation of a greater number of sperm than required to fertilize the ova. Both models and experimental observations indicate that males adjust the number of sperm in their ejaculate to the presence of rival males. Here, we show that Drosophila melanogaster males increase sperm allocation immediately after perceiving the presence of other males, but not females. Consistent with previous reports, we show that males use both auditory and olfactory modalities to determine the identity of potential rivals in their environment and we further show that these modalities are required for males to modulate sperm allocation. Our results support the sperm competition risk assessment theory, which predicts that males increase sperm allocation while perceiving the immediate risk of sperm competition, and reconcile previous observations in D. melanogaster that were at odds with this model.