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HUMAN sperm cells have an easily recognized tadpole appearance and, while not exactly iconic in society, have their own modest place in kitschy, not to mention downright tasteless, objets d'art. There are neckties with stylized sperm cells, salt and pepper shakers, and of course the inevitable coin bank in the shape of a sperm cell (get it?). At the American Society for Reproductive Medicine's 2008 conference, sperm cell-shaped USB drives were handed out. But such aggrandizement aside, the truth is that human sperm cells have a pretty humdrum appearance compared with those of many insects.

In his original paper, Parker noted that sperm within an ejaculate must compete, not only with sperm from rival males, but with each other, and therefore any attribute making one individual cell better able to succeed should be subject to selection. This variability is particularly important in the insects, because sperm are usually not used to fertilize the egg immediately, as is the case in many other animals, but are stored for a period of weeks, months, or even years before they are used. This means that anything giving a sperm cell longer life or a competitive edge over the long term will be valuable. Indeed, insect sperm morphology is amazingly varied, including some with multiple flagellae, the whiplike organs used to propel the sperm through the medium. Sperm cells are much more variable across species than other kinds of cells, and even more variable than many body parts; one could, at least in theory, use sperm characteristics to distinguish species, the way that beak shape or feather color are used by bird-watchers to determine whether they are seeing a black-headed blue warbler or a white-eyed vireo. This is probably unlikely to catch on as a pastime ("Hey, guess what—I spotted a double-flagellated big head over the weekend!"), but it points to an often unconsidered source of biodiversity.

Some species of the humble fruit fly are the real sperm champions, at least if you think size is what matters. Male Drosophila bifurca look pretty much like any other fruit fly, namely, tiny and brown. But they have sperm cells that are about twenty times the length of the male producing them. To put this into perspective, for a human male six feet tall to achieve a similar feat, he would have to produce sperm cells that could span a sizeable portion of a football field, to carry on with the sports analogies that inevitably seem to accompany discussions of sperm competition. The cells are mostly tail and initially are in tangled coils resembling balls of yarn, so that the males employ what one scientist calls a "peashooter effect" to get the sperm transferred to the female.

Needless to say, manufacturing such behemoths is energetically costly, and a male can't produce nearly as many so-called giant sperm as the ordinary variety. Male D. bifurca therefore "use their sperm with female-like judiciousness," according to Scott Pitnick from the State University of New York at Syracuse. Unlike most species of insects, including other types of Drosophila, D. bifurca males and females mate with roughly similar numbers of partners. The precious cells are produced on demand, with more being manufactured when males are given greater access to females and fewer when mating opportunities are scarce.

The function of the elongated tails is unclear. Some researchers suggest that they may block other males' sperm from getting through the female's reproductive tract. Alternatively, large sperm may have evolved because of selection by females for the more exaggerated forms of the cells, making them what Pitnick and his colleague Gary Miller called "the cellular equivalent of the peacock's tail." Pitnick and Miller took laboratory populations of D. melanogaster, a more commonly used fruit fly than the giant sperm-bearing D. bifurca, and subjected them to artificial selection experiments similar to the ones that Simmons employed with the dung beetles. Here, instead of constraining the number of mates an individual had, Pitnick and Miller