Sexual selection drives the rapid evolution of reproductive traits and genes. Specifically, high levels of sperm competition (the process in which female promiscuity forces the ejaculates from rival males to compete to fertilize the same ova) results in an improvement in sperm competitiveness. One of the main determinants of sperm competitiveness is swimming velocity, which enables sperm to outcompete rival sperm in the race to fertilize the ova. The increase in sperm metabolism associated with enhanced swimming speed is expected to increase levels of oxidative stress. Oxidative stress is known to affect sperm membranes as well as DNA integrity, leading to decreased sperm function and the transmission of genetic disease to offspring. We will investigate, in a group of closely related species of rodents that differ dramatically in their levels of sperm competition, if sperm competition results in higher levels of oxidative stress, and if such oxidative stress is prevented by the evolution of protective counter-measures such as changes in the composition of the plasma membrane, increased antioxidant defenses, or a greater degree of condensation of the DNA within the sperm head.