Friday Weird Science: The Armadillo Penis, a study in flexural stiffness

Aug 02 2013 Published by under Friday Weird Science, Uncategorized

You know, it's nice to know that there are people out there who really CARE. Who CARE about things like penises. Animal penises. And how they work. And who care so much about animal erections that they will create mathematical functions to describe them. Where would we (or at least, weird science) be without these people? I really cannot say. And most particularly, where would we be without Diane Kelly. Without Diane, who I would hereby like to promote to the Queen of Animal Peen (in an importantly non-dirty way), there would be no awesome studies of the armadillo penis, for example.

But we have Diane, and I am grateful. Because now, my friends, now we know all about the flexural stiffness of the armadillo penis. I'm sure that question has been plaguing you for ages, right? Right??

Kelly, D. "EXPANSION OF THE TUNICA ALBUGINEA DURING PENILE INFLATION IN THE
NINE-BANDED ARMADILLO (DASYPUS NOVEMCINCTUS)" The Journal of Experimental Biology, 1999.

Nine-banded_Armadillo
(I must say these guys are cute. Would make such charming little pets! Source)

The best thing about this paper is perhaps the opening sentence:

Before a male mammal copulates, his normally
inconspicuous penis changes both its size and its resistance to
bending or ‘flexural stiffness’.

I just can't get over the face that she called male penises "normally inconspicuous". I love it. I'm going to use it as an insult in bars.

But she raises an important question: males have erections, until the penis is stiff enough to fit inside the vagina without bending. The stiffness is due to the corpus cavernosum, a pair of spongy, tube shaped structures that run the length of the penis. What's interesting about the corpus cavernosum is that they are made of spongey material. You would THINK they should be made of hard material. But if they were made of, say, bone, the penis would be erect all the time. Instead, the corpus cavernosum is spongy, and fills with blood during erection. But how does it become stiff?
(A diagram showing the corpus cavernosum and the blood vessels that fuel it's...power. Source)

The answer is collagen. The outside of the corpus cavernosum is made of collagen fibers. But they aren't arranged like you would expect them to be. In other tube structures that need to stretch, collagen is usually arrayed in spirals, which can then stretch out as needed. But in the mammal penis, the collagen is arranged at right angles. There's collagen surrounding going front to back, and then collagen surrounding in loops. Like this.
(Figure 1)

All of that structure does look very rigid...but it doesn't look very FLEXIBLE. So how does it expand?

It turns out that the crossed structure is usually relaxed, with "crimped" collagen. The whole thing only goes straight and tense during the erection. As blood flows in, the collagen straightens out, and the crossed structure gives it "flexural stiffness", making it pretty tough.

But in order to look at the physics involved, Dr. Kelly needed...some penises. Preferably whole ones. Luckily, penises are easy to find, if you don't mind some roadkill. She picked up 28 male armadillo roadkills (ok, 26, 2 were donated by hunters). Using them, she sliced and diced some penile tissue, and examined how it worked.

She found that during erection, the armadillo penis increases by 26% in length and 17% in diameter. But that's just on the outside. On the inside, the corpus cavernosum increased by 50%, and the thickness of the coating around it of collagen (the tunica albuginea) decreases by 39%. Everything is stretching to accommodate all the blood, and the net increase is around 78% for a cross section (I don't know if humans increase that much, maybe human men would rather be armadillos?).
(Figure 3. Left is flaccid, right is erect)

Kelly also found that as you artificially inflate the penis (with air), the stiffness of the collagen fibers increases by 3-4 orders of magnitude! Stiffy indeed.

But the question is: why does the shape remain a TUBE? If all the collagen expanded equally, erections should be rounder. Not tubular (Incidentally, "The Tubular Erections" would be a great name for a band). But it turns out that penises have trabeculae, little rod shaped collagen structures, that go crosswise inside the corpus cavernosum and keep it relatively in shape. If you cut them out (and she did), you get a much rounder shape. So for a penis-shaped penis, you can thank your trabeculae. And for your stiffness you can thank your collagen. Who knew you could get so much from a roadkilled armadillo.

Kelly, D. "EXPANSION OF THE TUNICA ALBUGINEA DURING PENILE INFLATION IN THE
NINE-BANDED ARMADILLO (DASYPUS NOVEMCINCTUS)" The Journal of Experimental Biology, 1999.

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