10 Worst Adaptations in the Animal Kingdom

Slug Mating
Banana slugs during what appears to be a gentler phase of mating. © Kennan Ward/Corbis

So far we've had some pretty staid adaptations (or lack thereof) that are either slightly inconvenient, possibly unnecessary or just in the way. Now we enter the world of nonhuman animals, which have adaptations so troublesome that they must burn with jealousy at the streamlined, well-designed human body -- wisdom teeth and all.

We begin with our sticky friends, the banana slugs. Number one, slugs are all hermaphrodites. No biggie, right? In some ways, that's an adaptation that's super easy -- everyone can mate with everyone, with no pesky civil union/marriage question! But in this case, it can get a little tricky.

When banana slugs mate, they both insert their penises into each other. But at the end of the deed, occasionally a slug -- or both slugs, simultaneously -- will chew each other's penis off. Why? Some researchers think they need to in order to separate. And no, the penis doesn't grow back [source: Miller]. But good news! Now they just use the female reproductive organs and carry on.