Mystery solved behind missing penis bone!

Ever wondered, why unlike most mammals, men have no bones in their penis? A new study explains the mystery of lost baculum (penis bone) in the human lineage tends to be monogamous-- where you are married to, or in a sexual relationship with, one person at a time.

The research, published in the Royal Society journal Proceedings B, shows that the ancestral mammal, like humans, did not have a baculum - but both ancestral primates and carnivores did.

The baculum 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.

A new University College London study examined how the baculum evolved in mammals and explores its possible function in primates and carnivores - groups where many species have a baculum, but some do not.

The study may also provide clues as to why humans do not have a baculum.

"Our findings suggest that the baculum plays an important role in supporting male reproductive strategies in species where males face high levels of post-copulatory sexual competition," said first study author Matilda Brindle.

"Interestingly, humans have neither prolonged intromission durations, nor high levels of post-copulatory sexual competition. Given the results of our study, this may help to unravel the mystery of why the baculum was lost in the human lineage," Brindle explained.

In species where mating occurs between multiple males and females (known as polygamy), there is acute competition between males to fertilise a female, however, human mating systems are not like this.

Instead humans tend to be monogamous or, more rarely, polygynous (where one male mates with multiple females). In these circumstances, only one male has access to a female and post-copulatory competition between males is absent or very low level.

The work uncovers that the baculum first evolved in mammals between 145 and 95 million years ago.

"After the human lineage split from chimpanzees and bonobos and our mating system shifted towards monogamy," said co-author, Dr Kit Opie.

"This may have been the final nail in the coffin for the already diminished baculum, which was then lost in ancestral humans," Opie added.