Increased sexual activity reduces male immune function in Drosophila melanogaster

Despite the benefits of resistance, susceptibility to infectious disease is commonplace. Although specific susceptibility may be considered an inevita ble consequence of the co-evolutionary arms race between parasite and host, a more general constraint may arise from the cost of an immune response. T his "cost" hypothesis predicts a tradeoff between immune defense and other components of fitness. In particular, a tradeoff between immunity and sexua lly selected male behavior has been proposed. Here we provide experimental support for the direct phenotypic tradeoff between sexual activity and immu nity by studying the antibacterial immune response in Drosophila melanogast er. Males exposed to more females showed a reduced ability to clear a bacte rial infection, an effect that we experimentally link to changes in sexual activity. Our results suggest immunosuppression is an important cost of rep roduction and that immune function and levels of disease susceptibility wil l be influenced by sexual selection.