How Did Man Lose His Penis Bone?

The walrus penis bone in the Grant Museum is often pointed at with a titter, a gasp, and other whispered noises. That’s obviously not surprising – it’s longer than my thigh. Conversations normally go something like this:

Visitor [blushing]: I didn’t know that there was a bone in them.
Staff: Ah well, there isn’t in humans, but most mammals have them. There’s a few in a jar over here [points to jar].
Visitor: Why?
Staff: Mostly it’s about stamina.
Visitor: I feel sorry for the girl walrus.
Staff: Over here is a skeleton of a raccoon with its penis bone in position – you don’t often see this because the prude Victorians got into the habit of removing them out of common decency. There are drawers and drawers of them in store at big natural history museums.
Visitor: Gee whizz. So why don’t humans have one?
Staff: Good question [branches off into that kind of babble that professional communicators use when they don’t know the answer, normally involving offering the visitor the opportunity to discuss what they think the answer is. You’ll note that if the staff member had known the answer, we’d have seen the topic arrive when the visitor asked “Why?”]
[Visitor leaves]

Well, one of the wonderful PhD Student Engagers we employ to talk to visitors about their research and experiences of academic life, Suzanne Harvey, has made our lives much simpler by writing a blog which answers the question – How did man lose his penis bone? It’s over on their “Researchers in Museums” blog and it begins like this…

The walrus penis bone, also known as an os penis or baculum, is one of the most popular objects at the Grant Museum. The human penis is haemodynamic, meaning an erection is achieved by blood pressure alone. In animals with an os penis, blood pressure still plays an important role, but the pressure functions to push a bone structure into the penis in order to achieve an erection. This has many benefits over an erection sustained by blood pressure alone, not least in keeping the glans open for sperm to pass through.

While the importance of shaft size and sperm competition has been discussed in previous my previous blog post, even the largest penis will offer no evolutionary advantage if sperm cannot escape: these much desired qualities will never be passed to offspring. This is not the only benefit. The os penis increases the potential duration of intercourse and also the frequency with which intercourse can take place. For example, a lioness can copulate 100 times per day, sometimes with only four minute intervals, but has only a 38% conception rate1 – males need to keep up if they’re to achieve the best chance of paternity. It comes as a surprise to many people that the os penis exists at all, but in fact humans, woolly monkeys and spider monkeys are the only primates to lack this handy piece of anatomy.

You can read the rest of Suzanne’s post on the Researchers in Museums blog here