Imagine a sperm that’s the size of an apartment block. Fortunately, those don’t exist, although male fruit flies are packing the equivalent inside their sexy parts, with reproductive cells measuring a whopping 6 centimeters (2.4 inches) in length. That’s 20 times the size of the flies themselves.
Coiled up into a knotted ball, these gargantuan sperm unravel once inside the female reproductive tract, and have developed in order to provide individual males with a selective advantage over their rivals. The logic behind this quirk of evolution is explained in a new study in the Proceedings of the Royal Society B.
According to the study, the fact that smaller organisms tend to have the largest sperm corroborates the long standing sperm dilution hypothesis. This theory holds that, as the size of a species increases, so does the size of the female reproductive tract. Consequently, it becomes increasingly beneficial for males to generate larger numbers of small sperm, since this provides the best possible chance of one of them beating the others to the egg and fertilizing it.
Yet none of this would be necessary if it weren’t for female promiscuity. In other words, females are able to mate with multiple partners within a short space of time in order to improve their chances of becoming pregnant. This means that each male needs to produce the maximum number of sperm in order to ensure one of theirs beats those of their rivals.
However, in small creatures such as the fruit fly, it makes more sense to produce small numbers of larger sperm, since these can effectively block the relatively small female reproductive tract, preventing any rivals from getting their seed in there.
The long and short of all this is that mice have larger sperm than elephants, and if fruit flies ever grew to human proportions as a result of some diabolical scientific experiment gone wrong, their reproductive cells would be the size of the Statue of Liberty.