Like singles on the prowl, sperm seem to evoke an "every man for himself" attitude. New research, though, shows sperm can benefit from helping each other out. Sperm from a wild mouse band together and swim faster than individual sperm. Perhaps the first example of mammalian sperm cooperation, the finding may cause scientists to look for coordination among all types of sperm.
If a female mates willy-nilly with many males, sperm from her partners must compete for the egg at the end of the tunnel. However, because sperm from one individual are closely related, some researchers suspect they might work together towards the common goal. In some mollusks, for example, one giant sperm carries hundreds of other sperm through the reproductive tract of the female, dropping off sperm cells along the way. In South American marsupials such as the opossum, two sperms linked at the head zip faster than single sperm. Until now, researchers hadn't found examples of cooperating sperm in mammals.
Reproductive biologist Harry Moore at the University of Sheffield, United Kingdom, happened to notice that the wood mouse flaunts exceedingly large testes for its size. Because this is a telltale sign of fierce sperm competition, Moore investigated further. First, his team looked at sperm development in the testes. The sperm cells there sported an unusual hook that was folded down against the head, like a pompadour hairstyle. In mouse ejaculate, the individual sperm cells deployed their hooks and grabbed onto other sperm cells, forming large trains of sperm. The sperm stayed clumped in groups of 50 to thousands of cells for about an hour before dispersing, Moore and colleagues report in the 11 July issue of Nature. Timing the cells swimming through fluid, Moore's team discovered that sperm bundles motored almost twice as fast as individual cells.
There may even be self-sacrifice. Moore's group found that more than half the train-bound sperm prematurely burst their acrosomes--sacs containing the enzymes that break down the outer coat of an egg. Moore suggests that this is an altruistic action that breaks down the clump of sperm, allowing one to fertilize the egg.
"It's very unusual to find sperm helping one another," says primate biologist Alan Dixson at the Center for the Reproduction of Endangered Species in San Diego. However, he suspects that sperm cooperation is not unique to this one mammal. "People will be tempted to look more closely for coordinated behavior in spermatozoa."