Why Don't Humans Have A Penis Bone? Science Answers Again!


Erections may be commonly known as "boners," but anyone with even a basic understanding of human anatomy will know that men's penises don't actually have bones. In this regard, humans are different than many other species of mammals, including our closest relatives - chimpanzees and bonobos. Here is a possible answer to why it is so.

The males in most mammal species, including cats, dogs, and rats, have a bone in their penis called a “baculum,” or “os penis.”

Depending on the animal, bacula range in size from under a millimeter to nearly a meter long, and in shape, varying from needle-like spines to fork-like prongs.

This photo shows a 24 inch long Walrus baculum.

The walrus baculum, which could easily be mistaken for a 2-ft-long club (60 cm), is around a sixth of its body length, whereas the diminutive centimeter-long baculum of the ring-tailed lemur is only around one-40th of its body length.

Bacula are found in certain species of mammal, but not all.

Most primate males have a baculum, so humans are rather an oddity in that they don’t have one. In a handful of extraordinary circumstances human males have formed bones in the soft tissue at the end of their penises, but this is a rare abnormality, rather than a baculum.

Matilda Brindle and Kit Opie from the University College London set out to reconstruct the evolutionary story of the baculum, by tracing its appearance in mammals and primates throughout history.

According to the study, the baculum first evolved in mammals between 145 and 95 million years ago. It has been described as "the most diverse of all bones," varying dramatically in length, width, and shape in the male mammals where it is present. It was present in the first primates that emerged about 50 million years ago. So human males must have lost the bone in the evolutionary process.

So, why on earth would an animal need a bone in their penis in the first place?

Scientists have come up with a few hypotheses as to why a baculum might be handy. In certain species, such as cats, a female’s body doesn’t release its eggs until she mates, and some argue that the baculum may help to stimulate females and trigger ovulation. Another one is the vaginal friction hypothesis. This essentially argues that the baculum acts as a shoehorn, enabling a male to overcome any friction and squeeze himself into a female.

Finally, it has been proposed that the baculum helps prolong intromission, otherwise known as vaginal penetration.

Far from simply being a nice way to spend an afternoon, prolonging intromission like this is a way for a male to prevent a female from sneaking off and mating with anyone else. On top of this, males of primate species with longer intromission durations tend to have far longer bacula than males of species where intromission is short.

Another interesting discovery is that males of species facing high levels of sexual competition for females have longer bacula than those facing lower levels of sexual competition.

But what about humans? If the penis bone is so important in competing for a mate and prolonging copulation, then why don’t we have one?


According to scientists, 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. 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.

“We think that is when the human baculum would have disappeared because the mating system changed at that point,” Opie said. “This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans.” 

“With the reduced competition for mates, you are less likely to need a baculum,” he added. “Despite what we might want to think, we are actually one of the species that comes in below the three-minute cut-off where these things come in handy.”

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