Could a fire-breathing animal ever exist?

In George R.R. Martin’s fantastical land of Westeros in Game of Thrones and House of the Dragon, the spectacle of dragons breathing fire captivates his audience through a blend of myth and fantasy. For me at least, there’s also scientific curiosity.

The images of dragons unleashing torrents of flames on the new series of House of the Dragon got me thinking: if dragons existed, what real-world biological mechanisms and chemical reactions might they use?

But first, a chemistry recap. To ignite and sustain a flame, we need three components; a fuel, an oxidising agent - typically the oxygen in the air - and a heat source to initiate and maintain combustion.

Let’s start with the fuel. Methane could be a candidate. Animals produce it during digestion. The images on the screen of Westeros show dragons are keen on eating sheep. However, our methane-fuelled dragons would need to have a diet and digestive system more like that of a cow to produce enough gas to burn down a city.

There’s also a problem with the storage of sufficient amounts of methane gas. A typical methane cylinder might be rated for 150 atmospheres of pressure, while even a bloated gut can only tolerate a little over one atmosphere. So there’s no biological basis for non-marine Animals to store gasses under high pressure.

A better option would be a liquid. Ethanol could be an option. Maybe our dragons hold a vat of fermenting yeast in their guts, or they could have a metabolic system similar to Devil’s Hole pupfish, which live in hot springs in Nevada, US. Under low oxygen conditions, these fish switch to a form of respiration which produces ethanol.

However, storage is once again an issue. Ethanol quickly passes through biological membranes, so keeping it at high concentrations and ready to deploy on the "dracarys" signal (which translates to "dragonfire" in the fictitious language High Valyrian) would require some otherworldly biology.