In species in which females mate with multiple partners, spermatozoa from one male may have to compete to fertilize the ova with spermatozoa from another male, but the biological details of this competition have remained unclear. In 11 July Nature, Harry Moore and colleagues at the University of Sheffield, UK, show that sperm from a single individual cooperate and help each other in an altruistic fashion to advance faster to the ova (Nature 2002, 418:174-177).

Moore et al. studied the common wood mouse, Apodemus sylvaticus, and observed that their spermatozoa joined together via hooked structures on their heads and swam en masse in a train, enabling them to progress at almost twice the speed of a single sperm. Dispersal of sperm trains was needed for fertilization and this was associated with a premature acrosome reaction that compromised the fertility of all but few, chosen...