If you’ve ever wondered why there wasn’t more bone to your boner, scientists finally have the answer.

The penis bone, also known as the baculum, first evolved in mammals more than 95 million years ago, according to new research from University London College. While most mammals today have a penis bone (including chimpanzees and bears), scientists say the trait appears not to have developed in humans for a number of reasons.  

“The common ancestors of both primates and carnivores had a baculum,” lead author of the study Matilda Brindle told The Independent.

“Humans are quite weird as we’re one of the few primates that doesn’t have one." 

Brindle said one of the main reasons men lost out on penis bones was because they didn’t have sex long enough to need them.

She said primates who mate for three minutes or more tend to have way longer penis bones than those whose vaginal penetration (or intromission) durations fall below three minutes.

“The human intromission duration tends to be below two minutes, which most people wouldn’t expect,” she said.

In contrast, the nocturnal lemur known as the aye-aye copulates for up to an hour and has a very long penis bone.

Longer penis bones were may also be noticed in species with seasonal breeding and polygamous mating systems, said Brindle.  

“Polygamous mating systems are where multiple males and multiple females all mate with each other, like in chimpanzees,” she said.

“While polygamous mating does take place among humans, it’s not common enough to necessitate the retention of the penis bone.”

Other reasons for the lack of a human penis bone, said Brindle, include the fact that the human mating system takes place throughout the year and tends to be less competitive than that of other mammals.

While chimpanzees tend to only copulate for around seven seconds at a time, they have a polygamous mating system in which females will copulate with numerous males a day. This combination has resulted in chimpanzees having very small penis bones paired with very large testicles.

“[Human] testicles are slightly smaller, which indicates we don’t have such a strong selection for males mating with multiple females at the same time,” said Brindle.

“That’s probably the thing that finally got rid of our baculum, that we have more monogamous mating systems."

h/t The Independent