Sperm number and velocity affect sperm competition success in the guppy (Poecilia reticulata)

While both sperm number and quality are now recognized to be important in determining the outcome of sperm competition, very few studies have experimentally assessed the influence of these two parameters simultaneously. We studied the effect of sperm quality and number on competitive fertilization success in an internal-fertilizing fish, the guppy (Poecilia reticulata), which is characterized by high levels of sperm competition. We artificially inseminated virgin females with varying proportion of sperm from two competing males, while holding constant the total number of sperm transferred to the female. Sperm morphology and sperm swimming velocity were also determined prior to insemination. The paternity outcome of sperm competition trials was assessed through molecular analyses of the resulting offspring using polymorphic microsatellite loci. We found that both sperm number and sperm velocity affected the outcome of sperm competition, with males that contributed more and faster sperm achieving a greater paternity share.