Despite being a playful term, it appears the word "boner" does have a sense of reality to it. The presence of a baculum, or a penis bone, has baffled scientists for decades. It can be as long as a finger in monkeys and two feet long in walruses, but it is completely gone in humans.

Why? Scientists are starting to get an answer.

According to The Guardian, scientists are starting to "map" out the evolutionary story of the baculum by tracing its appearances in mammals and primates throughout history. They discovered that the penis bone evolved in mammals more than 95 million years ago and was present in the first primates around 50 million years back.

Now, the baculum has become larger in some animals and smaller than others. The 10-kilogram macaque has a five-centimeter baculum. This is five times the size of the baculum in the collared mangabey, a slightly larger monkey.

According to Science Magazine, Kit Opie and Matilda Brindle of the University College London said the penis bone length was longer in males that engaged in "prolonged intromission." This means, the act of penetration that lasted more than three minutes, which could help males impregnate females while keeping her away from competition.

The penis bone attaches at the tip of the penis rather than the base, and it provides structural support for male animals that engage in these activities.

According to their study, the penis bone in chimps is no longer than a human fingernail. The tininess of the bone correlates with the very short "spell" that the male spends mating, which is seven seconds. In chimpanzee groups, females tend to mate with all the males in what appears to be a strategy to reduce the risk of her children being killed by the older males.

Humans may have lost their penis bones when monogamy emerged as the "dominant" reproductive tactic by the time of Homo erectus about 1.9 million years ago. Scientists believe that the males do not have to spend a long time penetrating the female as she is not likely to be "leaped" upon by other males. Of course, this is just a theory.