Penis bones can be found in all shapes and sizes across the class of mammals, from the curly raccoon bone to the baseball bat-like walrus bone.
But humans, among a handful of other mammals, do not have a baculum (penis bone). It’s never truly been figured out why, but fortunately, scientists from University College London have risen to the occasion, so to speak.
In a study recently published in Proceedings of the Royal Society B, they outline the evolutionary story of the baculum and explore the idea that humans lost the bone because of monogamous relationships.
The researchers say that the penis bone evolved between 145 and 95 million years ago in order to help mammals have sex for an extended time in animal populations with high levels of sexual competition. Simply put, the bone provided the structural support needed for the male to last longer. Stiff competition between multiple rival males meant that longer sex sessions helped deter other males from fertilizing their choice of female.
An array of baculum from a variety of animals. Didier Descouens/Museum of Toulouse/Wikimedia Commons
Humans therefore, might have lost their bone when monogamous single-partner relationships developed (for the majority of humans) around 2 million years ago. When a male and female only have sex with each other, postcopulatory competition between males is lower and the length of sexual intercourse doesn’t matter in a practical sense (so the scientists say). Shorter intercourse times meant males no longer required the bone for support and were able to successfully procreate without it.
Of course, later on, cultural and social factors developed, which meant a few seconds of sex was perhaps not ideal, even if it is pragmatic from an evolutionary standpoint.
“Our findings suggest that the baculum plays an important role in supporting male reproductive strategies in species where males face high levels of postcopulatory sexual competition," lead author Matilda Brindle explained in a statement. "Prolonging intromission helps a male to guard a female from mating with any competitors, increasing his chances of passing on his genetic material."
The study also revealed some fascinating insights into penis bone size as well. Chimpanzees and bonobos, two of our closest cousins, have a baculum that is only around 6 millimeters long. Chimps and bonobos also only have sex for an average 7 seconds and 15 seconds, respectively.
"After the human lineage split from chimpanzees and bonobos and our mating system shifted towards monogamy, probably after 2 million years ago, the evolutionary pressures retaining the baculum likely disappeared," co-author Dr Kit Opie added. "This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans."