Primate sperm surface molecules - outrunning the competition and sweet-talking the female Pascal Gagneux1 1The UCSD Project for Explaining the Origin of Humans, Glycobiology Research and Training Center, Department of Cellular and Molecular Medicine, University of California, San Diego Abstract Primate species exhibit a wide variety of mating systems that range from monogamy to multi-male, multi-female systems. The mating behavior of females in each species determines the level of sperm competition between sperm from different males as well as the possibility for cryptic (post-mating) female choice. We are using quantitative flow cytomtery to study individual energy production in large populations of primate sperm. We measure the negative membrane potential of sperm mitochondria using staining with a cationic lipophilic dye (JC-1) followed by flow cytometric measurement of orange fluorescence. We are also staining sperm populations with fluorescent lectins and antibodies, in order to compare the cell surface glycoproteins and immune modulating proteins between species with dramatically different mating systems. In a comparison of Homo sapiens and Pan troglodytes, we are finding higher levels of mitochondrial energy production in Pan troglodytes. This is also the case for a few Pan paniscus samples analyzed. We are also finding large differences in the type of carbohydrate chains decorating the sperm membrane proteins and lipids between Homo and Pan species. Our results indicate that female -mediated sexual selection can cause rapid phenotypic and metabolic changes in sperm of closely related species.