There's a chance you're only alive today because the sperm you began as poisoned its competitors in the race to fertilise the egg.
Scientists have found that some sperm contain a genetic sequence which allows them to release a signal which sends other sperm swimming around in circles, while the lucky carrier races ahead.
"Sperm with the t-haplotype manage to disable sperm without it," Bernhard Herrmann of the Max Planck Institute for Molecular Genetics in Berlin said, referring to the piece of DNA which they've linked to the in utero trickery.
"The trick is that the t‑haplotype 'poisons' all sperm, but at the same time produces an antidote, which acts only in t-sperm and protects them. Imagine a marathon, in which all participants get poisoned drinking water, but some runners also take an antidote."
So far the t-haplotype variation has only been found in mice, but it's believed something similar probably exists in humans - we're both mammals, after all.
The poisoning is so successful, Dr Herrmann says, a mouse with the t-haplotype will pass it on to 99 percent of its offspring.
"Our data highlight the fact that sperm cells are ruthless competitors," said Dr Herrmann.
"Furthermore, the example of the t-haplotype demonstrates how some genes use somewhat dirty tricks to get passed on. Genetic differences can give individual sperm an advantage in the race for life, thus promoting the transmission of particular gene variants to the next generation."
The researchers hope their findings will one day help explain some causes of male infertility.
The study was published in journal PLOS Genetics last week.