Quantcast
Connect with us

How penis bones help primates win the mating game – and why humans might have lost theirs

Published

on

One of the most weird and wonderful products of evolution is the penis bone, or baculum. The baculum is an extra-skeletal bone, which means it is not attached to the rest of the skeleton but instead floats daintily at the end of the penis. Depending on the animal, bacula range in size from under a millimetre to nearly a metre long, and in shape, varying from needle-like spines to fork like prongs.

ADVERTISEMENT

The walrus baculum, which could easily be mistaken for a 2ft-long club, is around a sixth of its body length, whereas the diminutive centimetre-long baculum of the ring-tailed lemur is only around a 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.

In a new study, published in Proceedings of the Royal Society B, my colleague Kit Opie and I examined how the baculum developed in mammals by studying how it is distributed across different species in light of their pattern of descent (known as phylogenetics).

We showed that the baculum first evolved after placental and non-placental mammals split, around 145m years ago, but before the most recent common ancestor of primates and carnivores evolved, around 95m years ago. Our research also shows that the common ancestor of primates and carnivores had a baculum. This means that any species in these groups without a baculum, such as humans, must have lost it over the course of evolution.

So, why on earth would an animal need a bone in their penis in the first place? Scientists have come up with a few theories 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, somewhat colourfully named, theory 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.

ADVERTISEMENT

Is that it?
Shutterstock

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 before his sperm have had a chance to work their magic. This theory brings a whole new meaning to the term “cock-blocking”.

We found that, over the entire course of primate evolution, having a baculum was linked to longer intromission durations (anything over three minutes). 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 was that males of species facing high levels of sexual competition for females have longer bacula than those facing lower levels of sexual competition.

ADVERTISEMENT

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? Well, the short answer to that is 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.

But bonobos only copulate for about 15 seconds at a time and they still have a baculum, even if it is very small (about 8mm). So what makes us different? It’s possible that this comes down to our mating strategies. Human males (generally) have minimal sexual competition as females typically only mate with one male at a time. Perhaps the adoption of this mating pattern, in addition to our short intromission duration, was the last straw for the baculum.

ADVERTISEMENT

Scientists are only just beginning to piece together the function of this most unusual bone. What seems to be clear is that changes in the primate baculum are driven, at least partly, by a species’ mating strategy. The picture that seems to be emerging is that, under high levels of sexual competition, bigger is better when it comes to the penis bone.

The Conversation

Matilda Brindle, PhD candidate on the London NERC DTP, UCL

ADVERTISEMENT

This article was originally published on The Conversation. Read the original article.


Report typos and corrections to: [email protected].
READ COMMENTS - JOIN THE DISCUSSION
Continue Reading

Breaking Banner

Two impeachment articles expected against President Trump: reports

Published

on

Democrats are expected to announce on Tuesday two articles of impeachment against Donald Trump, US media reported Monday evening, after laying out their case at a hearing against a president they branded a "clear and present danger" to national security.

The articles will focus on abuse of power and obstruction of Congress, The Washington Post said, citing three official familiar with the matter.

It added that the full House of Representatives would vote on the articles next week, ahead of a trial in the Senate.

CNN said a third article on obstruction of justice was still being debated, and the network's sources cautioned that plans were still being finalized.

Continue Reading

Breaking Banner

Ambassador McFaul ‘shocked’ Trump invited Sergey Lavrov back to the Oval Office: ‘What are they thinking?’

Published

on

Former Ambassador to Russia Michael McFaul repeatedly said he was shocked that President Donald Trump will meet with Russian Foreign Minister Sergey Lavrov on Tuesday.

McFaul was interviewed Monday evening by Lawrence O'Donnell on MSNBC "The Last Word," where he contrasted how Trump is treating the Russian government of President Vladimir Putin to the Ukrainian government of President Volodymyr Zelensky.

"Ambassador McFaul, I want to get your reaction to the Russian foreign minister meeting tomorrow at the White House, in the Oval Office, with President Trump," O'Donnell said. "That's his second time. President Zelinsky still hasn't gotten that meeting and Donald trump apparently, apparently may be voted articles of impeachment in committee this week because of his interactions with President Zelensky."

Continue Reading
 

Breaking Banner

House Judiciary to vote on Thursday to impeach Donald Trump: report

Published

on

Democrats are moving ahead with the impeachment of President Donald Trump following another day of testimony on Monday.

"House Democrats plan to unveil at least two articles of impeachment Tuesday, charging President Donald Trump with abuse of power and obstruction of Congress, according to multiple lawmakers and aides. The Judiciary Committee plans to vote on the articles on Thursday, setting up a vote on the House floor next week to make Trump the third president in history to be impeached," Politico reported Monday evening.

"Democratic leaders plan to formally announce the articles at a press conference Tuesday morning. Judiciary Committee Democrats intend to meet ahead of the announcement and review the articles," Politico reported. "The decision to move forward with specific impeachment charges is the most significant move yet for the year-old Democratic House majority, a legacy-defining moment for Speaker Nancy Pelosi that sets up a Senate trial for Trump in early 2020."

Continue Reading