Os penis or penis bone or baculum.  Male cats have it, as well as male dogs.  As a matter of fact many male mammals do.  The walrus has an impressive one, reaching up to 22 inches in length.  But why don't humans have it?  Scientists say because humans don't need to have as much sex.

Men Tend To Be More Monogamous Than Primates, So They Don't Need A Penis Bone

Researchers at the University College London say that humans do not have the 'most diverse of all bones' because we tend to be a monogamous species.  They were curious as to why human's ancestral primates and carnivores have a baculum and men don't.

What they uncovered was that the baculum first evolved in mammals between 145 and 95 million years ago.  It is said that humans may have penis bones, but must have lost it over the course of time.  Prolonged penetration during sex - defined as longer than three minutes - correlated with baculum presence across the course of primate evolution.

Study lead author, Matilda Brindle, said: "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.  Prolonging intromission helps a male to guard a female from mating with any competitors, increasing his chances of passing on his genetic material."

Purpose Of The Penis Bone Is Said To Be For Longer Sex

Experts are not entirely sure about the purpose of a penis bone, but they said it could be to support the penis during sex, allowing it to have sex for a longer time.  Researchers found that polygamous species have much thicker penis bones than monogamous ones. 

Brindle said that  "humans don't quite make it into the 'prolonged intromission' category. The average duration from penetration to ejaculation for human males is less than two minutes."  As human are expected to be monogamous, and that our mating life is not as competitive as other primates, scientists theorized that humans gave up the need for the bone.