In a new study published in Ecology Letters, researchers set out to prove that it was something in the flies' semen — not their genes — that caused the size change. “The entire ejaculate has a whole bunch of other things in it; only 5 percent is the sperm itself,” lead study author and University of New South Wales post-doctoral researcher Angela Crean told Popular Science. “The sperm is what fertilizes the egg, but you have all these sugars and proteins and fluid that carry that sperm." And it's possible that some of those molecules are designed to affect female reproduction.

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To test the theory, Crean and her colleagues produced large and small male flies by way of feeding, then mated them with immature females. Later, these females were mated again — and this time, fully mature, they produced offspring.

But the kids didn't look much like dad: Even though the second male had provided the genetic material, the next generation of flies favored the size of their mother's first mating partner. Even though the genetic material of those first males hadn't fertilized the eggs, the special compounds in their semen (the actual composition of which is still unknown) may have gotten through.