Previous thread, same topic.
As for fire ‘breathing’ itself, I think a two step process along the following lines is plausible:
STEP 1: The development of a workable flame weapon.
*) A creature spits to defend itself. There are already plenty of creatures that already do this, from snakes to camels.
*) The spit becomes increasingly noxious by the addition of volatiles: ketones, alcohols, terpenes. Once again, the addition of noxious volatiles to defensive body secretions is widespread, from skunks to bombadier beetles.
*) At some stage the addition of volatiles produces a flammable mixture. Not a huge leap.
*) The addition of acids from the stomach produces an exothermic reaction capable of igniting the mix. Such reactions aren’t instantaneous, but if you pour a strong concentrated acid onto a wad of grass or paper it can combust within seconds. Since the stomach is already handy to the mouth and already produces a nicely noxious acid this step would seem almost too obvious NOT to evolve.
*) Our animal now has the equivalent of naplam in it’s armoury: a wad of sticky, flammable material that bursts into flame a few seconds after it leaves the animal’s mouth.
All the steps so far are easy to produce by simple evolutionary development. Far simpler than what we see with the bomabadier beetle. And we now have a camel that spits napalm. From that point we have a workable basis for developing classical fire breathing.
STEP 2: The development of true fire breathing
- We have a mechanism that will promote the evolution of heat-proofing the mouth (to protect from occasional premature ignition).
*We have a basis for the evolution of the ignition system (the napalm system is more effective if there is no delay while waiting for a chemical ignition). Something as simple as a row of hairs could provide a static spark that could ignite volatiles.
- We have workable basis for the development of specific flammable fuels. Alcohol would be a good choice since it is already genetically available to all tetrapods due to the enzymes that metablise alcohol catalysing reversible reactions.
*We have a workable basis for development of a flammables production system. The salivary glands in ruminants like camels have already been modified to store and secerete nitrogenous wastes, and the glands in various reptiles store and secrete numerous non-salivary enzymes as venom. It’s a plausible step to develop glands to store and secrete alcohol.
- By increasing the size of the salivary glands we now have a workable fuel storage systems.
At this point we can actually discard the precursor system of producing napalm. Our animal can spray alcohol directly at a target and then ignite the stream mid burst. The alcohol will immediately vapourise and hey presto:
WE HAVE ACHIEVED FIRE BREATHING.
Well actually we have achieved a primitive flame thrower, but the practical distinction between this and fire breathing is non-existent.