Drosophila bifurca, the fruit fly, has the largest sperm in existence: two inches long if the coiled sperm were straightened out. That is 1,000 times the size a human sperm. Biologist Scott Pitnick and Ph.D. student Adam Bjork of Syracuse University explored how such a sperm could exist, particularly in so small an organism. Their discovery, as seen the June issue of Nature, indicates that it has to do with the body of a female fruit fly.

In many species, far more sperm is produced by the male than eggs by the female in order to increase the chance of conception. As sperm is so much more common then eggs, male competition for females increases. The sexual selection theory states that the sperm to egg ratio balances out as competition among males decreases.

However, when the fruit fly species was closely examined, male competition had not diminished. This has to do once again with the females bodies. Even after the male has inseminated the female, fertilization is a long way off. Once the males have competed to inseminate a female, the sperm must compete among themselves in what’s known as post-cryptic choice. In this case, an extremely long reproductive tract—which is designed to act as a competition of endurance for the sperm attempting to fertilize the egg—which, is what led to the extreme length of the sperm, and which are almost as long as the tract itself.