The animal kingdom is full of examples that illustrate just how tough the mating game can be. But there is perhaps no better example of this than the female butterfly, which has evolved an entirely distinct organ to protect her own interests—in response to a similarly selfish evolution by the males, naturally.
Female butterflies have notoriously complex and convoluted reproductive systems, which include a relatively mysterious organ known as the bursa copulatrix. Recent research by University of Pittsburgh graduate student Melissa Plakke helped to illuminate what's going on in there, but to understand it you first have to understand the harsh game of strategy that is butterfly sex.
To help further their own lines, male butterflies—like many other insects—have evolved to deliver their sperm inside a package called a spermatophore. Spermatophores contain proteins to help improve their sperm's chances by giving sperm a speed boost, or by plugging a female's vagina so other males can't copulate with her.
This could potentially be bad news for the female, who would like to give herself as many chances as possible to reproduce and pass on her genes—which is why female butterflies have evolved the ability to dismantle those proteins and use them to maintain their own bodies and eggs. Because this is an evolutionary arms race of sorts, the outer shell of the male's spermatophore is surprisingly tough.
This back and forth is where the bursa copulatrix comes in. Scientists initially thought the organ might be "digesting" the tough spermatophore shell and trapping useful proteins, but they weren't sure. Plakke's research studied the interior of the bursa copulatrix in cabbage white butterflies, using specimens that were sexually active and some that weren't. She and her team found nine different enzymes in the bursa that break down proteins—meaning the bursa copulatrix is "eating" the male's proteins like a second stomach. Because adult butterflies don't actually consume other proteins, this organ has been compared to the section of the caterpillar gut responsible for digesting protein—but it's just one-twentieth of the size, which means the bursa copulatrix is just as, if not more, digestively powerful.