The University of New South Wales evolutionary biologist had mated average-size neriid fly females with either unusually large or small males, and he had expected to see offspring that were larger or smaller than usual. (Read more about flies' preference for semen.) 

Except he didn’t—the baby fly sizes were all over the map. 

After poring over the data, the only factor that could explain his findings was that the females had been mated before.  

Something in the original males' seminal fluid—the fly’s semen minus the sperm—had to be somehow altering the size of the female's subsequent offspring. 

Given the importance of sperm, it’s not surprising that scientists have focused much of their attention on these little swimmers. (See "Sperm Tracked in 3-D—A First.") 

But Bonduriansky believes that semen is also a major factor in mating and reproduction in a variety of species, from