Fast swimming sperm risk longevity and may not last until the egg arrives.

Fast swimming sperm risk longevity and may not last until the egg arrives.

It is a finding that could warm many men's hearts: size does not count and a paunchy middle is an advantage.

And in this competition, those with more sexual partners win the race.

Sperm have an average lifespan of several days inside the female reproductive tract. But if some swim too fast, they may exhaust themselves before the female egg is released, he said. 

Australian researchers studying how quickly sperm can travel inside the female to reach the egg have shown that having more than one partner is the key to faster sperm, in mice at least.

Leigh Simmons, an evolutionary biologist at the University of Western Australia, said that many animal species are polygamous.

This means an individual male's sperm has to compete inside the female reproductive tract with sperm from other successful suitors.

His team found that sperm from promiscuous mice swam 5 micrometres a second - or 0.00018 km/h - faster than sperm from monogamous males.

''Monogamous sperm have no selection pressure because at least some sperm will get up there [to the female egg] no matter how fast they swim,'' said Professor Simmons, whose findings are published in Biology Letters.

While the speed difference does not sound like much, it could be the difference between fertilising an egg or not.

Once the researchers established who were the faster swimmers, they set out to find what it was that gave them the edge. ''We were interested in whether longer sperm are able to swim faster because they have longer tails.''

But it is not the length of the tail that matters, it is the size of their mid-section, Professor Simmons said.

The mid-section - a powerhouse full of mitochondria, or energy organs, fuels the sperm, pushing them harder and faster towards the finish line.

Professor Simmons said it was the competition between other male sperm that caused this bigger midsection to evolve.

But while a little extra speed gives some sperm a winning edge, it comes at a price: longevity.

Sperm have an average lifespan of several days inside the female reproductive tract. But if some swim too fast, they may exhaust themselves before the female egg is released, he said.

But it was not all bad news for sperm, said Professor Simmons, adding that he believed a balance would develop between the two characteristics.

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