Sperm-storage organ of a female Drosophila bifurcaSCOTT PITNICKWhen it comes to reproduction, conventional wisdom has long said: sperm are plentiful and cheap, but eggs are rare and expensive. But the fruit fly Drosophila bifurca produces sperm that are 2.4 inches long, or nearly 20 times the male insect’s body size. (As The Atlantic noted: “If a man produced sperm that big, it would stretch diagonally across a basketball court.”) It turns out, longer sperm from many males compete to fertilize the female’s eggs by a form of post-copulatory sexual selection, Scott Pitnick of Syracuse University in New York and colleagues reported yesterday (May 25) in Nature.
To solve the mystery of why the flies’ sperm are so enormous, Pitnick and colleagues analyzed the shapes of male ornaments in other animals, including deer antlers, beetle horns, and peacock feathers. All of these pale in comparison with the D. bifurca sperm, which grow 5.5 times faster than the male flies’ bodies. “Biologists don’t think about these [gargantuan gametes] as weapons or ornaments, but they absolutely are,” Pitnick told The Atlantic.
By analyzing the evolution of this sperm and the female fly’s labyrinthine reproductive tract (which, in D. bifurca, is more than three inches long), Pitnick’s team was able to show that the female organ evolved to favor longer sperm—making it a form of sexual selection, even though the female has no real “choice.” ...
