Tiny mussel-like creatures living 100 million years ago made giant sperm longer than their own bodies, proving size has always mattered for some animals when it comes to sex, scientists said on Thursday.
Giant sperm are still around today. A human sperm, for example, would have to be 40 metres long to measure up against a fruit fly's. The insect is only a few millimetres in size but can produce 6 cm-long (2.5 inch) coiled sperm.
Scientists have been unsure if such gigantism is a freakish one-off.
Now the discovery that ostracodes, an extinct ancient class of arthropods, displayed the same trait shows that making giant sperm is a long-standing and evolutionarily successful reproduction strategy.
"Giant sperm have been produced in at least some species over long periods of time, even though they come at an extremely high price for both males and females," said Renate Matzke-Karasz of the Ludwig Maximilian University in Munich.
In most animals, including humans, reproductive success depends on males producing a large number of tiny spermatozoa, while females invest in a few large eggs.
But in some cases where sperm have to compete inside a female's body, the chance of successful fertilisation can be improved by increasing the size of the sperm cell.
Matzke-Karasz and colleagues used a new imaging technique known as 'holotomography' to detect organs used for transferring giant sperm in fossil remains of the ostracodes, which are only 1 mm long.
They described their findings in the journal Science.