It can be as long as a finger in a monkey. In the walrus, it can be two feet long. But the human male has lost it completely. And researchers are a little stumped.
Known as the baculum to scientists with an interest, the penis bone is a marvel of evolution. It pops up in mammals and primates around the world, but varies so much in terms of length and whether it is present at all, that it is described as the most diverse bone ever to exist.