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Remarkable Reptile #8
December 4, 2008, 7:38 am
Filed under: Uncategorized | Tags: , , ,

Sorry about the late blogging: I work on Wednesdays, and come home late, so the blog takes a bit of a back-seat priority. But, never fear: I still have, uh, forty minutes to write out a blog entry on my eighth remarkable reptile.

So, what do you get if you cross a cobra with a colugo?

Cape CobraColugo
Cobra + Colugo = ?

The answer, of course, is today’s featured reptile. Read on to find out …

The flying lizard, Draco is one of the most remarkable lizards in the world. It’s an agamid from the jungles of southeast Asia, and if you saw this lizard scampering in the branches and on tree trunks or flashing its dewlap in a territorial display, you might not think anything of it. That is, until you went to catch it, when a remarkable wing-like structure would unfold from the side of the lizard’s body, and the lizard would jump and then glide for perhaps hundreds of feet, coming to land a very safe distance away from your predatory intentions.

Draco volans with wings folded out

Draco volans with wings folded out

Okay, okay, I know what you’re thinking. Gliding isn’t really all that uncommon, not even among reptiles. There is a flying snake, a flying frog, flying geckos, flying squirrels, even a flying lemur out there. The ability to glide as a means of getting from one place to another has arisen multiple times in the animal kingdom, and while it’s certainly an interesting ability, it’s probably not enough to land an animal in the top ten category of remarkable diversity.

Well, that’s why I crossed my colugo with a cobra, you see. (To be fair, though, if I ever did a top ten in mammal diversity, I would definitely throw a colugo in.)

Because it isn’t just the fact that Draco flies that makes it amazing. It’s how Draco forms the membrane that keeps it aloft (known more formally as the patagium): and that mechanism is one of the most remarkable in the animal kingdom. Most gliding animals form their gliding structures with flaps of skin that either hang loose and unfurl under high air pressure, or are supported between struts of bone – for example, the patagium of a flying squirrel is formed by extending a flap of skin from the arm to the leg: when a flying squirrel spreads its arms, it creates a parachute that turns a free-fall into a glide. Flying frogs use their unusually large webbed toes to descend, and flying geckos use flaps on their toes, tails and side to create an effective parachute.

So, what does Draco use to create a gliding membrane?

The answer is, to me at least, completely astounding: ribs. That’s right, ribs. The same bones which, in you, create a nice, safe little nest for your heart and lungs, in Draco, have decided to forgo their normal function as body wall supports, and instead grow into that patagium, where specialized muscles can extend and retract them to create an airfoil. Now, this isn’t quite as absurd as the things some other reptiles do with their ribs – a similar system of flexible, modified ribs occurs in the hood of a cobra, where they are extended to create the hood’s iconic, flattened surface, and you’ll have to wait a few days to find out about what I think is probably the weirdest rib modification in the entire animal kingdom, but it’s nevertheless an extraordinary testament to the wonderful diversity of reptiles and the absolute weirdness of evolution that the structure was co-opted for such a unique purpose.

The following picture shows the undersides of Draco pretty well, where you can see the thin struts of the ribs, which push out to form the gliding surface.

Draco volans, preserved specimens

Draco volans, preserved specimens

Now, I could talk about how the tradeoff between pregnancy and flight has changed the morphology of females, or how gliding impacts agamid social behavior, but it’s late, and I need to get up early tomorrow. So, I’ll compromise by sending you over to a fabulous Animal Planet clip of Draco which fabulously illustrates this adaptation and the lizard’s real-life gliding behavior.

I’ve just blogged an animal that flies with its ribcage. If you aren’t starting to agree with me that reptilian diversity is marvelous and fascinating, there’s probably no hope for you. But maybe tomorrow’s critter will change your mind. And, whoops, this officially thirty-five minutes late. Sorry about that. You may now flog me.

Relevant Papers:

Patagial morphology of Draco volans (Reptilia: Agamidae) and the origin of glissant locomotion in flying dragons
Anthony P. Russel and Luke D. Dijkstra, Journal of Zoology, April 2001

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