Sperm competition is now recognised as a potent selective force shaping many male reproductive traits. While the influence of sperm competition on sperm number is widely accepted, its effects upon sperm size remain controversial. It had been traditionally assumed that there is a trade-off between sperm number and sperm size, so that an increase in sperm number would result in a decrease in sperm size, under conditions of sperm competition. Contrary to this prediction, we proposed some time ago

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... hat sperm competition favours an increase in sperm size, because longer sperm swim faster and are more likely to win the race to fertilize ova. Comparative studies between species show that in many taxa such a relationship exists, but the consequences of an increase in sperm size may vary between taxa depending on the environment in which spermatozoa have to compete. We present new evidence showing that in mammals longer sperm swim at higher speeds. We also show that mean swimming speed is highly correlated with maximum swimming speed, so even if the fastest swimming sperm are more likely to fertilize, both measures are informative. When individuals of the same species are compared, ratios between the dimensions of different sperm components, as well as the shape of the head, seem better at explaining sperm swimming velocity. Finally, we show that mean and maximum sperm swimming speed determine male fertility. Other studies have shown that in competitive contexts, males with faster swimming sperm have higher fertilization success. We conclude that the available evidence supports our original hypothesis.