Researchers have long known that the peculiarities of the female fruit fly's reproductive tract are responsible for these enormous sperm, which take a huge amount of energy to produce.

In that case, males should produce massive amounts of cheap sperm in order to have the best chances of reproduction.

Pitnick and his colleagues bred multiple lines of fruit flies with sperm "tagged" by fluorescent proteins, so researchers could tell which sperm came from which flies.

In doing so, the researchers were able to determine the factors that influence when and how sperm are successful.

The findings also reveal that though female fruit flies "choose" sperm simply based on the size of their own storage organs, the process works much like the sexual selection that occurs for flashy male ornamentation such as peacock tails or deer antlers, the researchers said.

"When people think about sex differences, they should be thinking about not just plumage and antlers and courtship dances.

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