Forget the typical'tadpole-like'image-sperm comes in all kinds of diverse and elaborate designs. But why should it have evolved to be like this? Tim Birkhead explains the ways sperm struggle to reach the elusive egg. As an undergraduate, the mere mention of the word 'fruit fly', or Drosophila (to use its generic name), would send me to sleep, But when I later learnt about their sperm, I came to love these insects, One species, Drosophila hyedi, is a world-record holder with 58mm-long sperm (see 'd' opposite) -- the length of your little finger. If humans produced sperm of equivalent length, each would be nearly 40m long. At the other end of the fruit-fly spectrum is Drosophilapersimilis, whose sperm are just 0,32mm long, Why should two closely related insects have such monumentally different sperm.
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