Scientists in the UK may have now found a solution to the long time mystery as to why sperm cells move like tiny, surface-seeking missiles.
The answer may have implications in resolving fertility issues.
UK mathematicians Dr David Smith and Professor John Blake at the University of Birmingham said that multiple factors are at work for this kind of behaviour.
First is a weak fluid dynamic force that pulls the cells towards the surface.
However, the tails of sperm cells tend to end up closer to the surface than the head - causing them to swim away from the surface.
The final factor is the thrashing motion of the sperm's tail, or flagellum, which gives it a pitching motion as it swims, causing it to swim first away from and then towards a surface.
All these effects combined cause the sperm's swimming trajectory to tend towards a specific distance from the surface.
"We understand that problems with motility are a major factor, and often go along with poor sperm shape and low sperm number," ABC Science quoted Professor Rob McLachlan, director of Andrology Australia, as saying.
"Understanding the process of normal sperm motility will help us unravel motility problems in infertile men and impact on clinical practice in the future," he added.
Their results are published in the journal The Mathematical Scientist.