When sperm cells race to the grand prize of an unfertilized egg, the rule is always "every man for himself"or so biologists have assumed. But Harry Moore of the University of Sheffield in England reports that the sperm of the European wood mouse actually collaborate, linking together into a cluster to speed the trip to an egg. Traveling in a bundle of hundreds or thousands of cells appears to offer an aerodynamic edge: Moore has shown that sperm swimming together reach an egg almost twice as fast as those that go solo. He and his colleagues also find that certain sperm in each cluster are preprogrammed to sacrifice themselves, giving up their adhesion molecules (which are essential for egg penetration) to help break up the sperm cluster when it approaches an egg. The discovery is the first major evidence of altruism among mammalian sperm, but there may still be competition in the behavior. In female wood mice who mate with more than one partner, for example, the true test may be which animal's sperm can build the better cluster, rather than which one has the fastest or strongest individual cells.