Unlike in humans, where it's every individual sperm for itself in the mad dash to be first to fertilize the egg, in some animal species sperm cooperate to form swimming "teams" to improve the chances of success, researchers have found.

The sperm of many animal species can come together in cooperative groups, outswimming single sperm cells in the path to the egg, they say.

In deer mouse, a common small North American mammal, the sperm have flattened, paddle-like heads with hooks once thought to allow sperm to adhere to each other.

Some of these groups prove to be faster than a single sperm, but sometimes not, the researchers found, and some groups can barely move at all.

Harvard University researcher Heidi Fisher and her colleagues took to their microscopes to try and figure out why, and it's all down to numbers, she says; seven to be exact.

That's the optimum number for an effective "group swim," and such a group will arrive at an egg faster then either a bigger or smaller group, the researcher report in the Proceedings of the Royal Society B.

But how?

"We had some hints that cooperation was enhancing their swimming performance, but what we didn't understand was how it was helping," Fisher says.

"What we realized was that while their overall speed wasn't increasing at all ... the time it took them to go from point A to point B was decreasing -- they swim in a straighter line."

That's because each sperm cell can cancel out the natural oscillating movements of its neighbors, and the group's velocity in a straight line increases, she says.

That works best in a group of seven sperm, she says. A smaller group will still have a tendency to wobble and meander, while in a larger group -- deer mouse sperm can form groups as large as 35 individual sperm -- the individual members begin working against each other and the group's velocity falls.

How the deer mouse sperm manage to hit the perfect number as often as they do is unknown, the researchers admit.

How the sperm manage to hang on to each other in groups is also uncertain, they say; contrary to what they had at first assumed, the hooks on each sperm head are not involved.

"That's the next step," researcher Hopi Hoekstra says of planned further studies. "How do they actually clump and recognize each other? We have no idea."