Physicists studying the motion of sperm have found that the little swimmers flock together in tight groups if the surrounding fluid is "viscoelastic".

Such fluids - including the mucus which sperm have to negotiate - are not only thick but have some bounce and stretch; they spring back when disturbed.

Watching bull sperm swim in different fluids, the scientists found that this gloopiness caused them to bunch up.

If the liquid was thick or thin but not elastic, they tended to swim solo.