When sperm compete to fertilize available ova, selection is expected to favour ejaculate traits that contributeto a male's fertilization success. While there is much evidence to show that selection favours increasednumbers of sperm, only a handful of empirical studies have examined how variation in sperm form andfunction contributes to competitive fertilization success. Here, we examine selection acting on spermform and function in the externally fertilizing myobatrachid frog, Crinia georgiana. Using in vitro fertilizationtechniques and controlling for variation in the number of sperm contributed by males in competitivesituations, we show that males with a greater proportion of motile sperm, and motile sperm with slowerswimming velocities, have an advantage when competing for fertilizations. Sperm morphology and thedegree of genetic similarity between putative sires and the female had no influence on competitive fertilizationsuccess. These unusual patterns of selection might explain why frog sperm typically exhibitrelatively slow swimming speeds and sustained longevity.
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