The Biological (Im)Plausibility of Dragons

So I made an off-handed remark about the biological plausibility of dragons in one of my Game of Thrones posts, and got several comments, posted and via e-mail, essentially amounting to “Wait! I want to read that post!”. Ask and you shall receive: I called in some help from my friendly neighborhood developmental biologist blogger, and she gave me some background information to run with.

I’m going to get to the fire-breathing thing eventually, but I’m going to start with something that is easier to explain away both mythologically and possibly genetically: the fact that many western pictures of dragons seem to be six-limbed vertebrates: two arms, two legs, two wings.

Now, the obvious counterpoint to the above is that not all dragons, even in western depictions, have arms as well as wings. There are plenty of four-limbed dragons. Not only Chinese depictions, which don’t have wings, but also wyverns, which don’t have arms. The dragons in the HBO Game of Thrones television series fall into this latter category. But the fact remains that the traditional western dragon body plan is… a bit off for a vertebrate.

To explain why that is, I want to talk about how vertebrate body plans are organized, and how extra limbs can be formed. And for that I need to get into some definitions. We have three axes: dorsal/ventral, which goes from your back to your stomach, anterior/posterior, which goes from your head to your foot, and right/left, which (unsurprisingly) goes from your right to your left (while your outsides might be roughly symmetrical, your insides definitely aren’t). The anterior/posterior axis is controlled by a family of genes called Hox genes. Hox genes are conserved all the way to flies and worms, so they’re pretty universal (and obviously pretty flexible in terms of things like number of limbs). How this works is that vertebrates, and arthropods (like flies), are segmented: very early in development we split the embryo into something like stripes, and then you get some repeated and some different structures in each segment. So, for example, you get repeated structures like ribs and non-repeated ones like arms. But it’s the same family of genes whether you’re a person or a fly or a worm. So the master regulators of head-to-tail development don’t really care whether you have zero limbs, four, or eight. But other gene pathways, farther down the line, do.

And now we get to the cool genetics thought experiments. How would you get another set of limbs? One of the most obvious ways would be to duplicate a couple Hox genes. Hox genes are kind of cool in that they’re laid out on the chromosome in the same order that they get transcribed, from anterior to posterior. So maybe you get a gene duplication or two and end up with two shoulder/upper body segments. Of course, then you’d end up duplicating a lot more than just limbs: all the organs and such that go between the limbs would also be duplicated. Which would be a little bit awkward. So we can get more specific. Is there a master-regulator that says “put a limb here!” instead of one that says “this is about where your shoulders should be”? Yes. It’s called FGF-8, and in chicken eggs if you add in FGF-8 expression where it shouldn’t be, you get additional wings or legs. (Whether you get a wing or a leg depends on where the extra expression is — right back to hox genes) So maybe you get some kind of regulatory co-option event that allows FGF-8 expression in two splotches near the shoulder blades: one for wings and one for arms. Then you can start maybe grabbing other genes from other pathways to make the two limbs different from each other. Or something. There’s some talk that this could have been what happened to give a pterosaur with six limbs.

For more on what that would take, check out the cool experiments in chickens documented here.

One quick note before we get to fire: this is assuming that once you got wings and two arms and two legs, the wing area to body weight ratio and general bone density and such are enough to support flight. Which isn’t necessarily likely for something as large as an adult dragons The biggest flying bird is the wandering albatross, and it only weighs 46 pounds. One scientist in Japan calculated that the heaviest an animal could possibly be and still fly – as in, long soaring flights, not quick take-offs like a turkey – is 88 pounds. The size of a large dog. Certainly not the size of a pterosaur or a dragon. That’s because a heavier bird needs longer wings to support it, which need more muscle to flap quickly and navigate between air currents, which therefore adds more weight. Muscle strength grows with cross-sectional area, but weight grows with volume, so after a certain point the strength to flap fast enough to keep yourself aloft is outpaced by the weight-gains necessary to flap that fast. There’s a bit of a controversy there, and if you want to read more about it this is a good place to start. But regardless of the details of the calculation, a full sized dragon would take quite a bit of lift, and whether or not we can get there with muscle and sinew is yet another thing to consider.

Now, for fire. This is the biological mechanism my dev bio buddy suggested: let’s start from something like a spitting cobra. This seems nicer because it’s a vertebrate. We’ll have it spray something very flammable instead of the usual mix of neurotoxins, hemotoxins, and other nasty stuff used by cobras. Ideally, something vaguely like gasoline or lamp oil: hydrocarbon, liquid, highly flammable. And you’ll need to protect the dragon itself, probably with heat-resistant and fire-retardant scales around the mouth and face.

Believe it or not, that’s the easy part. We also need a spark. The best we could come up with was some kind of source of elemental sodium or magnesium that could be uncovered at the same time as the hydrocarbon spray. It would certainly spark, since most life is mostly water and sodium and magnesium both react strongly to water. And there’s maybe almost kind of some evidence for something similar evolving: certain wasps have zinc or manganese-coated ovipostors. But on the other hand, zinc and manganese are far less dangerous to water-filled life than sodium or magnesium. The dragon would need to keep its sodium reservoir in a totally oxygen-free, water-free pocket, very close to the source of the fuel. Possibly a lipid pocket: possibly in a bubble of something vaguely like gasoline or lamp oil.

Mix in a little water, for example from saliva in the mouth, and it all goes boom.

Of course, we’ve gone increasingly far afield: it would also mean that a relatively small injury to the dragon’s fire-breathing organ would cause the entire thing to ignite. Which might not be evolutionarily favorable to the dragon. But might be advantageous to any people who could get close enough.