Among mammals sperm competition leads to selection for increased sperm numbers but it is not known whether it also leads to changes in sperm size. Two contrasting theoretical predictions have been made. The first hypothesis relies on the assumption that there is a trade-off between sperm numbers and sperm size and predicts that, in species confronting sperm competition, there will be a concomitant decrease in sperm size as sperm numbers increase. In contrast, the second hypothesis suggests that longer sperm may outcompete rival sperm; if longer sperm may swim faster, they will reach the ova sooner and will be selected when sperm competition prevails. We tested these hypotheses in both primates and rodents. We report that males from species in which females mate promiscuously have longer sperm than species in which females mate with one male. In addition, we also found that sperm length is positively correlated with maximum sperm velocity. Our findings thus support the view that longer sperm may be adaptive in the context of sperm competition.