Book I, Chapter 21
The dragon hydrocarbon organ system is the defining feature of the draconic clade; even in flightless, obligate aquatic, or the occasional limbless breed, if they possess the hydrocarbon organ system, they are defined as dragons.
The organ system is estimated to have originally evolved during the Neogene Period between 18 and 24 MYA, based on genetic evidence from the draconic genome, although some estimates put its origin much earlier, during the Paleogene's Eocene Epoch, between 48 and 34 MYA. Originally an offshoot of an additional liver lobe among tunnel-dwelling reptiles, its original purpose appears to have been in protecting the proto-dragons from heavy-metal poisoning accumulated from the stones of their native environment. Later evolutionary adaptations shifted the liver lobe into a distinct organ of its own, capable of chemical synthesis of basic hydrocarbons in conjunction with symbiotic bacteria, hypothesized as a form of energy storage. Later mutations created the secondary organs in the hydrocarbon system that distill oxygen, and transport, store, mix and ignite the hydrocarbons for firebreathing. True flight in the draconic species only appeared after the evolution of the pressurized oxygen bladders adjacent to the respiratory and hydrocarbon systems; at rest, the dragon uptakes oxygen from the atmosphere and stores it under pressure in the bladder. This oxygen is then either used for fueling flight muscles or for igniting hydrocarbons for firebreathing.
— An Introduction To Dragon Biology, 17th Edition, Oxford University Press, 1793
Tropes That Appear In This Chapter:
- Call-Back: Astrid sights the stunts Hiccup pulled in the race with thawfest as why she worries for him.
- Cold Turkeys Are Everywhere: Heather feels massive guilt working as a spy for Adalwin. Then Fishlegs just walks up to her and introduces his and Hiccup's newly revised Book of Dragons that contains everything Adalwin would need to tame dragons.
- Friends Are Chosen, Family Aren't: Hiccup admits to Magnus that while he does not wish harm on Snotlout, he's more worried about Hookfang's safety than with Snotlout given their history.
- Hate Sink: Apparently Brogan is so grouchy, only Mildew and his cronies can stand him.
- Made of Explodium: The reason why miscarried dragon eggs explode is due to complications with bladders in dragon physiology that allows them to breathe fire. This is also why the Hooligans use nets to capture and later dispose of dragons when they were at war instead of using arrows and spears, as puncturing these bladders will cause them to explode and only cause more problems in the process.
- Red Herring: Fishlegs come to the conclusion that Brogan is the spy.
- Rewarded as a Traitor Deserves: Both subverted and defied. Mac Bethad had no intentions of having Mildew offed as soon as he got the eggs and dragons that he wanted, and Mildew specifically warns that he has precautions and contingencies put in-place.
- Sand In My Eyes: Feeling guilty, Heather rationalizes that the onions she's cutting is why she is crying.
- The Singularity: Downplayed. Wulfhild explains that by training dragons, Berk has inadvertently "upset the game board" as far as politics has concerned.Wulfhild: Now things have changed. Now, a tiny chiefdom has power, of the sort that make armies and titles and lands irrelevant. And, most importantly... the young man who truly rules that power is kind, and honest, and decent.
- True Companions: Wulfhild admits that, like Magnus, Hiccup and Astrid are the only real friends she's ever had, every other friend cozying up with her because they thought she would be queen and then abandoning her when Magnus took over.
- Ugly Cute: How Wulfhild describes Gronkles In-Universe.Wulfhild: They're cute, but in a way that's so ugly that they came around and crept in through the back door!