After animals have mated, sperm in the female reproductive tract must race to gain that incomparable prize, fertilization of an egg. Because sperm express little of their genetic make-up in their outward appearance, it is difficult to select the good from the bad. So it might be better for an individual's sperm to cooperate rather than compete with one another. For species in which females mate with multiple partners, this will be particularly true if the sperm of one male could unite to defeat those of its rivals.
Elsewhere in this issue, Harry Moore and colleagues describe an amazing example of such altruistic behaviour in the sperm of the common European wood mouse, Apodemus sylvaticus (Nature 418, 174–177; 2002). They find that hundreds or thousands of sperm link hooked structures on their heads and swim en masse in a train, which enables them to progress at almost twice the speed of a single sperm. These trains must break up before fertilization, so many of the component sperm commit genetic hara-kiri by undergoing a premature 'acrosome reaction'. This involves the release of enzymes that break down cell adhesion molecules, which also makes it impossible for the sperm concerned to fertilize the egg. Somewhere on the train — perhaps it's the locomotive driver up front — there must be one acrosome-intact sperm that has retained its capacity to perform fertilization.
It might be no accident that the wood mouse is a supreme sexual performer among rodents, with males scrambling to mate polygamously with any and every promiscuous female, and they have relatively large testes to prove the point — as the picture shown here attests. So an individual male not only tries to ensure his reproductive success through sperm collaboration, as Moore et al. show, but also by sheer sperm numbers.
The story does not end there. Sperm motility is ultimately driven by the engine of mitochondrial DNA in the sperm's midpiece. An exciting paper by Matthew Anderson and Alan Dixson (Nature 416, 496; 2002) has shown that, in primates, the volume of the sperm midpiece is highly correlated with relative testicular size and mating behaviour, the most sexually athletic species having the largest mitochondrial midpieces to power their sperm. So we can look forward to further work to see whether the wood mice have vast mitochondrial midpieces to power their sperm trains, and whether the sperm of especially promiscuous primates such as chimpanzees would leave their human counterparts for dead in the Olympic swimming pool.