Mouse sperm have been caught trying to poison their competitors to reach the egg first.
Scientists from the Max Planck Institute for Molecular Genetics in the German capital Berlin have analysed samples from rodents.
They found that some carry a genetic variant called the t haplotype, allowing them to move faster and in straight lines. Sperm without this variant from the same mice tended to swim less productively, often moving slower and around in circles.
The variant is considered to be a "selfish" genetic element, and researcher Bernhard Herrmann and his team have now discovered how these sperm gain their advantage.
Sperm cells carrying the t haplotype variant produce molecules that are able to disturb other sperm cells.
Experts hope this research will allow them to develop treatments for infertility in male humans.
The team also found a link between sperm success and a crucial protein in the body called RAC1, which plays a role in general cell movement and directs the sperm cell towards the egg.
Levels of RAC1 in the cells have to be just right – too high or too low and the sperm cells will not move straight.
If the levels of RAC1 are disturbed with, their internal guidance system fails, as do their hopes of reaching the egg.
It turns into a literal life or death race.
Dr Hermann said: "Sperm immotility is a big deal in male infertility. Investigating the levels of this protein in human samples could help to develop treatments for infertility in men."
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