Published online 11 July 2002 | Nature | doi:10.1038/news020708-10

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Sperm teams out-swim the competition.

Sperm groups travel roughly 50% faster than loners. © H. Moore

Mouse sperm team up to speed their route to the egg. New research shows that hundreds of individuals form fast-swimming packs to give their genes the best chance against other males' sperm1.

In the process, many sacrifice their own chances of winning the fertilization race.

"I've been studying sperm for 30 years, and I've never seen anything like it," says Harry Moore, who discovered the cooperative sperm by chance in the wood mouse (Apodemus sylvaticus). "I was absolutely amazed," he says.

One or two minutes after ejaculation the sperm link up head-to-head or head-to-tail, using a hook on their heads

"They're like the carriages of a train," says Moore, who works at the University of Sheffield, UK. In the lab, trains can swell to thousands of sperm. In female mice they tend to number 50-200 cells.

Groups travel roughly 50% faster than loners - especially in viscous fluid like that in the female reproductive tract.

Human sperm might also cooperate, says Moore. When they pass through the cervical mucus, he says, those at the front behave differently to those in their wake. "The leaders could be pathfinders that change the mucus to ease the passage of the rest," he suggests.

Heads up

Only individual sperm can enter the egg; wood mouse trains start to decouple about 20-30 minutes after forming. Many sperm - particularly those in the centre of groups - trigger a reaction in their head that is usually used to bore into the egg.

This reaction makes them less sticky, Moore thinks. Sperm that set it off prematurely blow their chances of fertilization.

This creates a potential disagreement between a male and his own sperm, points out evolutionary biologist Geoff Parker of the University of Liverpool, UK.

“There could be conflict over which sperm sacrifice themselves” Geoff Parker

University of Liverpool

"A male parent doesn't mind which of his sperm fertilizes the egg," he says. But every sperm wants to be the lucky one, although it would rather one of its brothers reached the egg than one from another male.

There could be conflict over which sperm sacrifice themselves, Parker speculates. This would depend on whether males mark sperm for doom before ejaculating, or whether the self-destruct sequence is initiated within the sperm itself.

Wood mice are promiscuous; sperm from different males often race each other to fertilize an egg. The cells go for strength in numbers as well as cooperation: testes take up nearly 5% of a mouse's body weight. Comparable human testicles would weigh about 3.5 kg.

Invertebrates also have collaborative sperm. Some molluscs, for example, produce different-sized sperm, with the small ones hitching a ride on their big brothers.

University of Liverpool