The mating habits of the earth's creatures are as varied as those creatures themselves. Some don't reproduce sexually at all, and those who do can surprise us. As with most lists, this one contains some things you've already read about at Neatorama, but not all of them. I bet you don't already know about barnacle penises.
Barnacles may have the most interesting penises in the animal kingdom. Their penises can stretch up to eight times the length of their body. And while many species' manhoods can change size, few can alter their shape. But researchers in Alberta found that barnacles that live in gentle waters have long and thin penises that are good for reach, while those in rough waters have short and wider penises that are better for holding off strong waves. The researchers then transported rough water barnacles to calmer waters and vice versa and found that after getting moved around, the barnacles adjusted their penis's shape to better fit their environment.