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.