Hydras in my world are much like a brontosaurus in form, but has three heads. Much like a hippo, hydras have incredibly dense bones, and sink rapidly in water. Hydras are almost exclusively swamp creatures, using their long necks (Which are like a large snake in shape) to keep above the water. Like a hippo, Hydras have hooves, (Despite being a reptile.) and have the mobility of a hippo underwater. Hydras have limited magic- the most ancient hydras can alter their own density slightly, and every mature hydra has access to a boiling acidic venom (It still must get into the bloodstream to have any effect.) they can spit at reasonable distances. (Non-magical spitting.) Hydras cannot fly. The most important aspect of the Hydra, is it's incredible regeneration. If a Hydra's head is cut off, it will grow 2 in the place of it, reasonably rapidly, (This is magic.) The energy does come from the hydra, so doing this enough will exhaust it, but it can use it's stores of fat and dense bone marrow to fuel this. My main concerns for this creature, is that it will just sink into the swamp, or be incredibly inefficient. Can the swamp floor support such a massive creature? Hydras can weigh over 50 tons, and if they grow too many heads without shedding, they can die. If a hydra would sink, is there any way to prevent this? Would adding two more legs still allow it to run as fast, or even solve the problem, if any? If it is impossible to not sink, how can a hydra escape the bog?

Other questions about hydras do not solve this, and I can't find anything on how things sink into the swamp, but I do know they do. Pygmy dinosaurs have supposedly been spotted in the Congo, so at least we know that type of creature can live in the swamp. (Maybe?)

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    $\begingroup$ to be fair hippo just sink, hippo are horrible swimmers, they are too dense. So they just run along the bottom instead, and like your creature they have hooves. $\endgroup$
    – John
    Commented Sep 22, 2023 at 0:06
  • $\begingroup$ Exactly the point of the hooves. $\endgroup$ Commented Sep 22, 2023 at 0:36
  • $\begingroup$ Not exactly the point of the question, so I'll just comment, but any substance that's boiling and acidic will have detrimental effects in contact with skin. Now this is the point of the question: Are you in love with the hooves? Because a flat, ray-like body would be a lot better in this environment because of larger surface area $\endgroup$
    – JB3
    Commented Sep 22, 2023 at 13:27
  • $\begingroup$ That Hydra can weigh over 50 tons sounds like a good argument for sinklng, but when you talked of brontosauruses and hippo forms, you didn't mention size. If these are four-footed critters presumably each foot carries about 50/4, so rather more than 12 tons weight, no? Then how far apart are those huge feet and prolly more importantly, what's roughly the area of each foot pad? How do your Hydra compare to elephants, at up to about seven tons or blue whales, being the biggest critters on Earth, at perhaps 200 tons? Under all that, what's the bog floor made of? $\endgroup$ Commented Sep 23, 2023 at 23:05
  • $\begingroup$ I have my answer. The Hydra sinks. Sometimes. It will be able to escape using the snakelike mobility of it's necks, and will run quickly- exploiting inertia to avoid the whole issue of sinking. As for the acid, The fangs that shoot out the boiling acid are protected against that. I assume that can be done without magic. However, Hydra skin is not immune to boiling acid. $\endgroup$ Commented Sep 25, 2023 at 6:05

3 Answers 3


That Sinking Feeling That Physics is Involved

50 tons is a lot of weight. Elephants clock in at a mere 5-6 short tons. Long-necked dinosaurs are around 17 short tons. This fantastic creature is sunk!

These animals can only sink so far: the Archimedes Principle applies here. You can assume mud and quicksand is liquid enough for this equation to apply! It is common enough practice in civil engineering applications to treat dirt and mud a bit like a liquid and it gets pretty good results.

All this to say the following things:

  • the hydra sinks until it displaces it's own bodyweight in mud/quicksand, likely not its whole body
  • if the body would be entirely submerged, it will hit solid ground "eventually" and can slowly work its way out. Hippos are actually so heavy so they can "trot" a long the bottom of water instead of actually swim. Same for this hydra, only it's swampy mud.

Long Necks Solve Issues

A hydra could rely on it's long necks in one of a few ways:

  • To ambush and eat food because the main body is stuck. The science term is "a feeding envelope," and was a reason why we think long necked dinosaurs evolved
  • To pull itself out (assuming snake-like mobility in the neck).

Which are good reasons to have long necks! More importantly, that main body should be a bit more smooth to help with the "pulling itself out" bit. Maybe hydras do not so much walk as drag itself about. More feet will help spread its weight out, but do cost a fair bit of energy!

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    $\begingroup$ Thanks for physics! Anyways, it makes sense, the more ancient, dormant, and large hydras spend long times resting, only subconsciously breathing and smelling for passing food- then ambush! Younger Hydras remain in the shallows or on the surface, using superior speed and momentum to hunt. Sinking might even be useful while dormant, as you might be able to kill a hydra with a severed heart. $\endgroup$ Commented Sep 22, 2023 at 0:44

If you are really aiming for scientific accuracy, be aware that in 1951 it was pointed out that water pressure would make it impossible for a sauropod to breathe under water. See this blog post for the whole story (which can be confirmed elsewhere.)

But being aware doesn't necessarily mean changing anything. This limitation only applies if you rely on air pressure to inflate the lungs, the way existing animals do. You could imagine (for instance) a very strong, appropriately built creature having muscles and bones arranged to be able to exert immense force against water pressure to allow air in. Not biologically likely, but not physically impossible either. So, no problem at all for a good fantasy!

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    $\begingroup$ Or maybe, something with two lung cavities, two airways, and a rigid outer skeleton. Then a diaphragm moves between them so while one airway has an inward flow, the other has an outward flow. Or, you could get really creative and have a hole in the diaphragm with a valve, so air is pumped in a consistent direction through the system, like a heart pumps blood. $\endgroup$
    – Azendale
    Commented Sep 22, 2023 at 19:42
  • $\begingroup$ I mean, it is a literal dragon. I'm not going to hand wave everything, but this? Yes, dragons are incredibly strong, and in my world, have literal angels for souls. Either the hydra is magically this strong, or is simply very well at dispersing oxygen into the body, maybe hydras make their bodies go dormant, requiring very little oxygen, while each head has enough oxygen intake to survive and breathe and hunt for the body. $\endgroup$ Commented Sep 22, 2023 at 21:10
  • $\begingroup$ Yeah, I revised my answer just to make it more clear that I'm totally on board with your approach. I just thought this was an irresistible bit of information in this context. $\endgroup$ Commented Sep 23, 2023 at 2:02

You can solve that with two simple questions about the bog.

  1. How deep is the bog?
  2. Does the bog have a soft bottom or a hard bottom?

If the bog is "deep" or "soft bottomed" then the hydra sinks too far into the muck.

But, if the bog is "shallow" & "hard bottomed" then you can have the hydra stand/walk etc on the bottom of the bog without sinking too far into the muck.

So you just have to explain that bog trait in story & say that hydras choose shallow, hard-bottomed bogs to live in.

That is what real life animals do, they choose den/nesting locations based on certain criteria to fit their needs. (take a look at bird houses & notice how many different designs there are which appeal to only certain species but not others, if birds can be that picky why not hydras?)

So this can help add realism: the hydras are limited in which bogs/areas they can reside in, there isn't a hydra lurking in every bog in your world.

You don't mention if hydras or good/heroic or bad/monsters, or how you otherwise intend to treat them in the story, but you can use the limitation of the hydra's habitat to your advantage.

Perhaps a character must go on a search for a hydra & first finding the right kind of bog is part of that challenge.

If your hydra is a monster, the hero could trick the hydra into leaving it's shallow bog lair and chasing him into a deep, soft-bottomed bog where it sinks & drowns. Or, if the hydra is heroic, a villain could attempt to trick it into drowning in a deep bog.

I'm sure you can come up with any number of ideas.

  • $\begingroup$ Useful answer. I have limited knowledge of swamps, I had assumed everything would be relatively 'boggy'. A Hydra is an intelligent species, as are all dragons. Dragons have literal angels for souls, but because of the 'flesh' aspect, can still do evil, and some dragons have fallen angels, aka demons for souls. Doesn't turn out too well. $\endgroup$ Commented Sep 25, 2023 at 6:01

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