The male fruit fly has giant testes. They account for 11% of his body mass. And for good reason. In most animal species, the female produces relatively few eggs and the male produces vast quantities of sperm, but the fruit fly does it a little differently. The male fruit only makes a few sperm cells in his entire life – but they are all whoppers.
So just how big are these sperm? Well, one individual sperm cell can reach 5.8 cm in length – and to put that into perspective, that’s a thousand times longer than any human sperm, and twenty times longer than the fruit fly himself. The sperm cell is almost entirely comprised of its long, whip-like tail, coiled up like a ball of string, which is why the fly needs such giant testes to store them.
Rather than blasting the female with many millions of sperm like most animals do, the fruit fly just unleashes one huge monster of a sperm cell. Due to its gigantic size, however, it’s fairly sluggish off the mark, so the female needs to use an internal transport mechanism within her body to enable it to reach its destination. Once inside the female, the sperm releases a chemical that drastically lowers her libido, ensuring that she doesn’t go eloping with other males immediately afterwards.
And now the question must be asked: why on earth would the male fruit fly produce such a ludicrously long sperm? Well, you can blame the female actually. She has a very long and complicated genital tract, which is coiled up like a spring in her abdomen, so your average minute sperm wouldn’t be able to get through it and reach her eggs. This complex, seemingly hostile genital passageway is actually a screening test – only the fittest and most nourished of fruit flies can produce extra-long sperm, so successful fertilization is a reflection of their physical prowess.