The penis bone can be as long as a finger in a monkey and two feet long in a walrus, but the human male has lost it completely. According to a new report published in Proceedings of the Royal Society, the lack of a penis bone in human males may be a consequence of monogamy and quick sex. The Guardian reports: 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. Prompted by the extraordinary differences in penis bone length found in the animal kingdom, scientists set out to reconstruct the evolutionary story of the baculum, by tracing its appearance in mammals and primates throughout history. They found that the penis bone evolved in mammals more than 95 million years ago and was present in the first primates that emerged about 50 million years ago. From that moment on, the baculum became larger in some animals and smaller in others. Kit Opie who ran the study with Matilda Brindle at University College London, said that penis bone length was longer in males that engaged in what he called "prolonged intromission." In plain English, that means that the act of penetration lasts for more than three minutes, a strategy that helps the male impregnate the female while keeping her away from competing males. The penis bone, which attaches at the tip of the penis rather than the base, provides structural support for male animals that engage in prolonged intromission. Humans may have lost their penis bones when monogamy emerged as the dominant reproductive strategy during the time of Homo erectus about 1.9 million years ago, the scientists believe. In monogamous relationships, the male does not need to spend a long time penetrating the female, because she is not likely to be leapt upon by other amorous males. That, at least, is the theory.