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A human sperm can move up to 70 per cent faster if it has a lazy tail, a finding that could pave the way for new fertility diagnostic tests.

Sperm cells use their tails to swim, though some don鈥檛 use the whole tail 鈥� leaving a piece at the end inactive. This part only constitutes about 3 to 5 per cent of a normal sperm tail 鈥� which is usually between 50 and 55 micrometres long 鈥� and doesn鈥檛 actively bend like the rest of the tail. But it may be key to gaining speed.

The tail makes a shape a bit like a sine wave to propel the sperm, says Meurig Gallagher at the University of Birmingham in the UK, who worked on the study. 鈥淭he tail moves left and right, but when you get to the end, that part is also trying to move this way in the fluid.鈥�

鈥淲e found that when the end piece instead relaxes with the tail 鈥� it generates a shape that allows the tail to swim more efficiently.鈥�

Gallagher and his colleagues, led by Cara Neal also at the University of Birmingham, devised mathematical models for how sperm swim. Unlike previous models, the team included the end piece of a sperm鈥檚 tail that had been historically overlooked.

鈥淣obody has looked at the end piece because it鈥檚 effectively at the limit of light microscopy,鈥� says Neal.

The researchers modelled sperm swimming in a range of environments, including in semen and in the female reproductive tract, including in cervical mucus. They found that sperm with an inactive end piece swam more efficiently and faster than sperm with tails which were completely active.

Depending on the environment, this type of tail was found to propel sperm 20 to 70 per cent faster and was between 1.5 and 4.5 times more energy efficient when swimming.

The team suggests that sperm with an inactive or relaxed, tail end swim quicker because this results in the tail taking up a more efficient shape.聽The researchers hope this finding could help doctors determine why someone is fertile or infertile on a more detailed level.

鈥淭he truth is we don鈥檛 know how important this finding will be in real terms, but it is great that they are doing this type of work, as it has clearly been neglected,鈥� says Christopher Barratt at the University of Dundee, UK.

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