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In my fantasy world there's the race of trolls, vastly inspired by the ones one encounters in Baldur's Gate 2 (see here).

Millennia ago, they've been driven underground by orcs and elves, as well as occurring humans later on due to lack in numbers and technological advancement. The dwarves at first tolerated their presence but as the trolls are mesmerized by the sparkle of gold and the resulting raids on dwarven mining sites they drove them back to the surface in a couple of millennia. Now the trolls live in a huge cave system in a mountain and the close outside valley bordered by a river in the west and another mountain range in the north keeping among themselves and travellers off.

They're 2.5 to 3 m (~8'2'' to 9'10'') tall if they stood upright and despite their somewhat sleek look, they have stronger bones and muscles than humans to support the increase in weight. As they spend most of their time in more or less tiny caves they adapted quickly to a bent walk setting them at eye level with humans at most.

They also have a highly efficient regeneration allowing them to regrow severed limbs including the head in mere hours for use and fully in a few days. Even without head their heart keeps pumping and as long as their blood doesn't run cold and doesn't clot their regeneration works. One can slow it down by the use of fire or acid as the fire closes the wound (the key to the regrowth of limbs on salamanders is that their wounds don't close) and the acid eats into the flesh destroying regrowing cells. This is but a temporary solution. Driving a blade into the heart of a troll also can't kill it, as minor wounds get healed quickly enough for not requiring the heart to beat and keep the blood flowing. The only way to finally kill a troll is to cut out his heart since this is the only way to stop the regeneration ability powered by the remaining blood as it takes too long to regrow a heart and thus the blood runs cold.

Is this somewhat plausible to not explain it with "magic"? Is such a high regeneration possible? Could it be explained with an organ responsible for constant stem cell creation or anything like that? Can such a high regeneration ability be able to adapt their walk in, say, 10k to 20k years without causing chronic pain in their backs while also not decrease their size much? What about the regrowth of the brain, how could that be possible? I know I'd have to deal with memory loss. What about the energy consumption. Is it possible to regrow a head without being able to eat? Maybe leave it tiny at first with a very small brain and then regrow according to the amount eaten? Are stronger bones and muscles in combination with their rather sleek look enough of an explanation to make their body size possible? Is there anything else I missed that makes their physical presence impossible? Why do I have so many questions?

EDIT:

High regeneration is absolutely necessary. Their fighting strategy is to catapult themselves with copied ballistas from stolen dwarven blueprints over walls of cities ignoring all the resulting injuries. If they're not able to get back on their feet and start fighting within minutes to few hours, that strategy is doomed.

However, as this would result in bruises, broken bones and other inner injuries rather than cut off limbs, regrowth of limbs can take its time. Also, they do not have to regenerate fully very quickly but only so they're in fighting shape. As they're still taller, stronger and tougher they'd at least not be in disadvantage to humans. Also, surprise is on their side.

Now again the question, would that be possible? Imagine a person falling down a couple of stories on a plastered road, he's dead.

EDIT 2:

I'm trying to clump together what I got as answers and comments until now. Thanks to everyone so far.

The trolls stay slim with strong muscles and bones making them heavier but also reduce blunt trauma upon falling and give them slight resistance to flails and the likes. Their slimness partially compensates for that. Height also stays the same as nobody knows how large they've been before they were driven off underground, thus the occurrence of a slight reduction could have happened anyway. They have a hard time standing upright and usually do so only on rare occasions like intimidating others or taking a look over obstacles. They have a rather slow metabolism and generally lead a life with less movement than humans do. They train and enslave kobolds to gather food and do any other kind of work such a small being can handle, so the trolls themselves don't have to move as much. If they engage in physical activity they burn their energy quite quickly, so in combat they get exhausted somewhat fast making them a much less dangerous enemy afterwards. Don't get pleased too quickly though, you won't survive that long. This leads them to being great sprinters and jumpers with their long legs but earns them the last place in the New York marathon. Also, if necessary, they eat parts of their enemies to regain energy, they love the taste of human infants even more than the sparkling of gold.

Regeneration: Wounds without removal of body parts like stab wounds or broken bones regenerate rapidly at first. Blood clotting starts quickly and an effective stem cell creation and deployment process generates tissue closing open wounds and stitch together bones within minutes. The latter leads to skeletal deformations in numerous cases. Final healing process can take up days to few weeks depending on severity and requires quite some food and rest. In order to cope with the fast beginning of the healing process trolls have an energy reserve in form of a round belly. Refilling this reserve additionally delays final healing process. This, in combination with a vast distribution of endorphins to suppress the pain, lets them engage in combat incredibly quickly after experiencing physical trauma.

Regrowth: Fast plot clotting and stem cell creation helps with regrowing limbs as well and the process starts quickly, too. The belly fat pad does barely help though, so a disfigurement leads to the troll instinctively evade the battlefield, thus desertion is no rarity. Once out of danger the injured needs a lot of food and rest. Regrowth takes few months up to a year. If possible, hours to days later the body enters a form of hibernation, only interrupted to eat, speeding up the process vastly (maybe half the time?).

Beheading: Now to the fun part. Upon decapitation the body immediately shuts down any non required functionality. Loosely coupled brain cells scattered across the torso adopt control of the organ functionality, which is absolutely required to keep the body alive. This makes the body nearly impossible to kill as you have to destroy it entirely in order to prevent it from functioning. Rudimentary redundant memory cells enclosed in the spinal cord store basic memories so the regrown brain doesn't start at zero, body functions like walking and important personal memories forming the basic character. It would be possible a beheaded troll becomes a different one if their "society" would actually encourage personal individuality. The scattered brain cells in the torso are also able to weakly process optical stimulus, so the first things to regrow are eyes and mouth so the troll can identify, consume and process food to gain energy. From then on the hibernation state mentioned earlier is the way to go with the full development of the brain being the last to occur, as it takes up to several years to fully function correctly. In the process, the body can manage more and more activity so the sleeping phases of the hibernation state shorten after some months. In the early time the troll behaves very animalistic only trying to satisfy the most basic instincts.

In all three cases the blood stream takes control of the stem cell distribution, so the troll can't regenerate anymore when you cut out his heart. Also, before engaging in combat they eat as much as they can to gain a highly possible energy buffer.

Does that sound plausible now? The only problem I currently see is the energy consumption until the mouth has developed. What about photosynthesis and water? Could that give the body enough energy to slowly heal if not doing anything else? How about collecting necessary nutrients through the skin?

EDIT 3: I've finally managed to write it all down and will mark Hankerecords' post as answer as it has helped me the most. Thanks to everyone for helping :)

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    $\begingroup$ Starfish regenerate limbs and take 6 - 12 months to do so. I imagine your best answer will significantly dial back the "hours to days" timespan for your regeneration. $\endgroup$ – Ranger Sep 7 '16 at 13:53
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    $\begingroup$ I've read a salamander takes only two weeks. The fast regeneration is essential though, so that's the question, how fast could it possibly be? I'm editing the question for you to see why. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 13:58
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    $\begingroup$ You can't rationalise that speed with real-world biology. It's magic. $\endgroup$ – John Dallman Sep 7 '16 at 14:21
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    $\begingroup$ @OttoAbnormalverbraucher If I might say, maybe if that's the major problem you're asking the wrong question. You want them to be able to fight even after they've been thrown with a ballista/catapult, so you're asking how they could regenerate from the injuries. What if they were able to avoid getting injured in such a fall though? $\endgroup$ – Hankrecords Sep 7 '16 at 14:56
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    $\begingroup$ I can only imagine two possibilities. Either they're all flabby like rubber or have a very strong and hard shell. The former would simply be weird, and I'd assume not even possible outside of water, and the latter would overpower them too much. Maybe a combination of both, regeneration and toughness is possible? They have stronger bones and muscles anyway. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 15:46
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This is something that does not and cannot exist in our world, most definitely. Lizards and salamanders can regrow their tail, but it takes a lot of time (several weeks) and a lot of energy and, as the wikipedia article says, the new tail is not made of bone but of cartilage. Of course, this is unpractical for our trolls, so they would need to regrow bones. That would take an awful lot more time, at least with a biology similar to those we know. So I would say that such a regeneration could only be explained by magic or, at most, by simply stating that their body acts differently than the human one. They would need a really slow metabolism (but highly efficient in terms of energy output) to survive while regenerating.

Some species have the ability to not only regenerate limbs, but also organs (like the starfish)

Some species, like the planaria, can grow an entire new body starting from a portion as small as 1/279th of their total size, but they are worms, nowhere near the complexity of 2.5-3m tall dudes.

The complete regeneration of their head might be feasible if they had some sort of system that resembles that of ovule fecundation, but rather than generating a new being generated just a brain. Of course, this would not only mean memory loss, but a 3m tall guy with the brain of an infant.

To make it possible for them to survive without a head, other than a really slow metabolism (and possibly a way to "freeze" their body, something similar to winter hibernation, like bears do), they would need their vital organs to be able to work without receiving signals from the brain. This is not possible in our body, but they may have some sort of "very small brains", one for each organ, that tells the organ what to do when it's not receiving any electric signal from the head. Of course, such a body would be a lot more complex than ours, since then each organ would have to be able to "communicate" not only with the brain, but with each of the other organs.

Or, you know, you can always say it's magic ;)

They would need a reeeaally efficient regeneration system to be able to regrow a limb in minutes, and they would need a f■ckload™ of energy to be able to do that, so, as said before, a very slow metabolism, and possibly an enormous stomach to store as much food as they can a couple hours before a battle.

If they only need to be fast at regenerating inner injuries, while limb regrowth can take time, it's more feasible, as they would "only" need a really good stem cell system that would stop blood loss really fast and completely seal the wound in minutes see this. I guess a species with a body capable of regenerating limbs, including their head, would be able to heal from inner wounds really fast without major problems.

As for the communication between the organs, you may even have an organ the only purpose of which is handling said communication, possibly placed in a protected area (behind the heart would be ideal, as that would mean that if this organ is injured then it means the heart has been taken out, and if it hasn't, than the "communications organ" would be able to regenerate). This way, the brain would only need to be connected to this specific organ, which would take all the decisions regarding organs, so that the absence of a brain would be irrelevant to the functioning of vital organs.

Regarding their posture, I think they might be able to adapt and avoid chronic pain after a few generations. As a result, though, they would lose the ability to stand up correctly over the time, and I think a loss in height would be inevitable as shorter individuals would be more agile in their environment and would live healthier lives altogether, increasing their chance of survival and their chance of carrying their genes on to the next generation.

I think it could be feasible for them to survive without a mouth, provided that the pharynx and larynx (and windpipe and throat) are the only open "passageway", as all the veins going to the head have to be shut down immediately after decapitation to prevent bleeding to death.

If the throat remains accessible, they may be able to ingest food in fluid form, or at least really soft (unless they have a really acidic stomach that can break down whole pieces of food without problems, which is entirely possible considering they don't need to digest quickly due to their slow metabolism). If the larynx and pharynx have been cut off with the head, for example, the regeneration process may "know" that, in case of decapitation, it needs to develop a sort of valve at the outermost part of the throat and trachea, to prevent them from ingesting or breathing unwanted "stuff". This valve could then be lost (and simply spit away) once the mouth is fully functional. In this scenario, they could eat a lot of food through this hole, hibernate and simply rest for a few weeks until the mouth is ready. For reference, see how cockroaches survive without head.

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  • $\begingroup$ Guess you misread the edit. They don't need to regrow limbs in minutes but they need to regenerate inner injuries quickly enough to be on their feet in minutes. Regrowth of organs and limbs can take time. About the brain part, what about brain like tissue spread across the torso, which is able to capture minor electric signals from the brain and at least keep the body and regeneration process alive? That'd make them very vulnerable to electricity, I guess. Well, they're more intelligent than an infant as they do have a rough language and culture but compared to a human they're quite stupid. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 14:30
  • $\begingroup$ @OttoAbnormalverbraucher Sorry, edited again. $\endgroup$ – Hankrecords Sep 7 '16 at 14:46
  • $\begingroup$ Not a thing, I'm glad people answer :) I guess an organ behind the heart would not survive a stab through the heart, thus removing the heart completely wouldn't be necessary. I'd rather stick to decentralized tissue spread across at least a large part of the body. Thanks for the link, gotta read through it later, though. They rarely stand up so the loss of that ability wouldn't be of any trouble. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 14:56
  • $\begingroup$ @OttoAbnormalverbraucher I figured the communications organ would be able to regenerate just like the other organs :) so a stab would not be enough. $\endgroup$ – Hankrecords Sep 7 '16 at 14:59
  • $\begingroup$ I thought if you need an organ to manage the regeneration process, you'd probably not be in a good situation if that organ itself got hurt during the time of regeneration. Maybe I misinterpreted your answer. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 15:09
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Bamboo can grow 35 inches per day. That's about 1.46 inches (3.7 cm) per hour. I think that's about the upper limit on the growth of a large complex organism.

There are many reasons why this is easier for a plant than an animal. Matter can be neither created nor destroyed. For an animal, food is broken down and basic nutrients are carried by the circulatory system to provide the energy and amino acids to create new proteins. An inert animal only has the resources it brought with it. In a plant, photosynthesis allows it to use primarily water and carbon from the air surrounding it in the continuous construction of new cells.

Additionally, a stalk of bamboo has really two purposes: structural and vascular. A cross section of bamboo basically has radial symmetry and looks the same no matter where you slice it. An animal's limb is made of more kinds of tissue. Muscle must be anchored to bone in some places and move freely in others. Joints require careful matching of physical features and a system of lubrication. In all cases the growth of one sort of tissue outpacing another could be disastrous, so you're limited by whichever component grows the slowest. Animal growth is just very carefully orchestrated by comparison.

Finally, animals have a much much higher metabolism, and that requires a much more active circulatory system. Insects lack a heart and you can see they are rather limited in size, yet trees grow to enormous size without a heart, because it's not really important that materials in their circulatory system reach their destination quickly. We also have a lot of specialized organs, while most of a plant's needs are satisfied by a few different kinds of tissues that are not grown in just one vulnerable point. All of this explains why my lawn can get run over with a mower and fare much better than I would. Your trolls would have to survive trauma in the first place to be able to heal or regrow, and you will be just as hard pressed to find an explanation for their durability.

You can make your trolls a bit more feasible by making them a bit more like bamboo I guess. Their excretory system could rely more on their skin, and liver and kidney functions could be performed by tissue just under the skin throughout their body. I guess your trolls would be quite stinky. They could be made of more flexible material than bone, like cartilaginous fish. They could be cold blooded and possess a very slow metabolism. That would limit them to very sedentary lifestyles, and fast movement in only quick anaerobic bursts. With reduced metabolic needs, a reduced blood flow might be accommodated by the valves that already exist in our blood vessels and rhythmic contractions throughout the circulatory system.

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    $\begingroup$ +1 in general, but especially the idea that the entirety of the circulatory system itself could function as a redundant (if lower volume) pump. $\endgroup$ – Michael Richardson Sep 7 '16 at 17:05
  • $\begingroup$ @MichaelRichardson Thanks. It already kind of exists but in a very passive secondary capacity for us. en.wikipedia.org/wiki/Skeletal-muscle_pump It could be greatly refined and refocused for an organism that wanted to become more homogenous for the sake of surviving injury $\endgroup$ – Stephen Lujan Sep 7 '16 at 17:56
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Healing takes energy. In the case of regrowing a missing part (compared to just repairing an existing part), you also will require mass to use to build the new part.

So, starting with the assumption that such regeneration is possible, your Trolls will need to have substantial energy stores that will be burned at a tremendously high rate when such rapid healing and regeneration is required. A troll that has recently required heavy healing and / or is malnourished, is simply not going to be able to heal at such a rate.

If you require that healing simply requires consuming mass quantities, then you have the additional time to break down the food before being able to use it to heal and regenerate.

As to the missing brain, their nervous system may be much more decentralized or redundant, or perhaps personality and memory engrams are encoded into the spine so that a newly grown brain can be given the patterns of thought and memory of the original brain.

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  • $\begingroup$ So, before being catapulted, they'd eat as much as they could in order to not have to find food to heal. How could such a substantial energy store possibly look like? Masses of fat would contradict their physique. Memory redundantly stored in the spine is a very intriguing idea. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 15:06
  • $\begingroup$ They should be grossly obese looking for just this very reason...but I am sure the poster thinks they should be all muscular and cut looking. But you can absolutely be strong and fat--plus, their size means that they will be stronger anyhow. Fat seems like a pretty decent way to store such a thing. It's certainly a start on an explaination, even if magic will still be needed. The lower the BMI, the less likely the troll can heal. $\endgroup$ – Erin Thursby Sep 7 '16 at 15:15
  • $\begingroup$ An image is attached ;) Yes, you can be strong and fat, but at the same time you're slow, which is not what is intended for the Trolls. No, they're not supposed to be all ripped but rather sinewy. I could change their physique to have a somewhat thick belly but nothing close to "grossly obese". That means they'd need a highly more efficient energy store than simple pads of fat. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 15:41
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A slightly sci-fi suggestion

If the trolls were, in fact, a collection of giant single cell creatures (up to a few inches or even a foot or so across) which have decent control over internal structures, and a limited ability to morph their external membrane, who have learned to mimic creatures around them, you could accomplish this.

Imagine: a society of ameboids, mostly animalistic or of low level intelligence, with the occasional "general" being born of high intelligence among them.

After years of predation, despite the ability to hide via mimicking, a general discovered that instead of eating other ameboids, it could form vascular and neural links with and then control other non-general ameboids. Once the linkages are formed, however, they cannot be severed without killing off the non-general ameboids.

With more mass, larger creatures can be mimicked, and even hunted. The general ameboid forms the central neural cluster and main heart. Part of the linkage process overwrites the neural pattern of the non-general ameboids, effectively lobotomizing it, which is why it can't survive on its own, nor regenerate another troll upon severing.

It could possibly make semi instinctive use of genetic material to learn how to grow new things (bones, claws, useful organs, and so forth).

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  • $\begingroup$ I like this one, although it's a little sci-fi-ish (as you've already pointed out). Reminds me of Mgalekgolo (Hunters) from Halo :) $\endgroup$ – Hankrecords Sep 7 '16 at 19:04
  • $\begingroup$ Cool idea, but far from physically possible. Also it would be more fragile, as a cell whose membrane is ruptured is effectively dead. Never let reality get in the way of a good story though. :-) $\endgroup$ – Stephen Lujan Sep 7 '16 at 19:34
  • $\begingroup$ Heh, neat. Not fitting into my world and barely explainable in a medieval/fantasy setting as nobody knows anything about ameboids or much about biology. Also, the trolls actually only play a minor role in the story, this sounds more like a major role. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 19:53
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    $\begingroup$ A lot of worldbuilding is simply explaining it to yourself, so that your imagination and common sense will stop fighting over it and move on to the next thing. The readers or players need never know these details, unless it becomes necessary, and even then you don't explain it in the science fiction way, you use words the locals and natures would know. This explanation was just one possible method. Look up the Dralasites for my inspiration. $\endgroup$ – nijineko Sep 7 '16 at 22:57
  • $\begingroup$ Very true. Though I thought about an attachment of explanation of some races, historical, cultural and biological already. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 23:08
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I'm going to focus on the goal, raining trolls, rather than the question.

To make it rain trolls, you need trolls that can fall a great distance without dying.

One method is regeneration. This is a traditional property for trolls, so makes a whole lot of sense, apart from that pesky science thing.

Another method is not breaking in the first place. SO many ways to do this.

Feather-falling (parachutes, wings, magic, super-low-density bodies, etc).

Hard-close-fitting shell and acceleration-resistant innards. Not OP as others claim because when they stand bipedal, they expose their soft underbelly, like a pillbug.

Airbags. Not sure how this could work.

Crumple zones. Maybe they have four legs, but only need two?

Fluid bones/cartilage, hardened by pressure or muscular action. This one seems possibly doable. It gets you a crumple zone/airbag effect in the limbs that can then, like erectile tissue or tentacles, reform to a hard column. It resolves the whole "fix broken limbs" thing. I suspect that the end result, if you did the math, would be "splat", though.

Super-heavy bones. Bones made to withstand impact. You'll rip open some flesh around the bones, perhaps, but the bones themselves are thick and yet flexible, more like steel than marble.

Landing on soft things. Target landings to hit people. Requires some level of aimed or directed flight. I just like this one because it seems most trollish.

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    $\begingroup$ Airbags ... I actually already thought about them taking enslaved kobolds with them on their flight so they could use these as "cushion" for a softer landing, heh. Aimed flight is not possible as they cannot see through the large city walls and thus have no idea where to shoot. Also the quality of their ballistas is actually kinda weak and they're not versed enough for aimed shooting. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 22:58
  • $\begingroup$ Ooh. I love the idea of the kobolds. I'd argue here for the Rule of Cool. $\endgroup$ – Dewi Morgan Sep 8 '16 at 20:24
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I think what you are looking for is the T-1000, not a troll.

But to get to your question in a more serious manner, how would you address the concern of restoring memory/intelligence in the case of a head that needs regenerating? I presume that's where the brain will reside. Even if you have several "mini-brains" to control the limbs and vital organs (octopi have something like this) there must be a central data repository, less those mini-brains aren't so mini and there is parity across the hive minds (brains in a RAID array?) Such a network of brains would require an immense amount of energy, but you already have an energy problem to solve given the whole regeneration thing.

Energy aside, without the ability to restore memory, when your trolls are catapulted into the enemy's fortress and suffer severe head trauma, they will awaken on the other side of the wall without a clue as to what happened. They'll be persuaded by the enemy to join their ranks and fight against you, only the enemy will be gentler to the trolls and won't catapult them so they won't forget who they're fighting for.

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    $\begingroup$ Love the idea of a RAID brain. $\endgroup$ – Michael Richardson Sep 7 '16 at 19:14
  • $\begingroup$ Well ... they won't get decapitated upon impact, so I don't see any problem with memory loss in that situation ;) It's perfectly fine if they "wake up" barely knowing what's going on but with few, most basic memories they could realize the "head trauma" and at least try to figure why it occurred. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 19:57
  • $\begingroup$ How about the rule applied to zombies: kill the brain, kill the troll. And to address the problem of the catapult strategy, they can do something like what NASA did with the Mars rovers and the trolls can wear a balloon suit of sorts, or maybe just a very cushiony suit, which they'll shed after they are on the other side of the wall. Or wear helmets. Brain regeneration just seems to go too far... $\endgroup$ – urbanite Sep 7 '16 at 20:32
  • $\begingroup$ The trolls actually aren't technically advanced. They're quite dumb and their culture is more tribalistic than anything else. In no way, they're able to develop balloon suits, gliders or any technical means of flying or landing safely. If it weren't for the dwarven blueprints they wouldn't even have ballistas to shoot themselves. And please, don't mix up brain regrowth with their catapult strategy. They are aware some don't make it on the other side, as the US army was aware "some" wouldn't make the normandy invasion. The catapulting is more connected to broken bones, inner bleedings and such. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 23:16
  • $\begingroup$ The beheading is a different matter and occurs on a different part of the story. One that leads to the invasion but during the invasion itself, no disfigurement is even described as it is just the setting but not the actual main plot. The important part is that the strategy works to get into a large scale battle in the city. $\endgroup$ – Otto Abnormalverbraucher Sep 7 '16 at 23:22
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Thinking about regrowing a head and brain... no, not at all possible. The brain (brain stem) controls all those picky little things like breathing and heartbeats, lose that and they won't survive enough to be regenerated... new personality is the least of their worries, it might be months for them to develop enough and re-learn enough to even breathe.

But, but - they're not human. A different species, even. So...why do their brains have to be in their heads?

I'm thinking the brain, or even just the brainstem, could be located in the trunk (for better protection) - maybe in the neck, or spine, or between organs in the torso, anywhere well-protected. Neck would be close by for migration, but still vulnerable to decapitation damage, while lower in the spine might be workable (and spinal column is fairly well protected). On the other hand, it might be hidden - under the curve of the hunched over spine, or even at the other end of the spinal cord form the neck. Few would be looking for a brain in the pelvis (or ass) and also, think of all the jokes. it has to be hidden or protected or both, since damage to this brainstem is still very, very bad - but if nerve or brain injuries can heal over time, minor-ish accidental injuries to it might be survivable (with many aftereffects). The "head", then, is just a limb wired with sensory capabilities, and perhaps extra brain matter for processing capability and memory storage (essentially, an external hard-drive) - especially if you want memory loss and so on for story-purposes.

It will take a lot of energy to regrow the head, with the added bonus of needing to energy (which means food) while healing and headless - perhaps if they're more social, they can expect a friend to be pouring (or poking) food down their belly while they're incapacitated - a simple valve system (either quickly produced at cut point, or at regular intervals anyway) would let the throat breathe and possibly swallow without the mouth. On the other hand, if they need to be more self-sufficient - they might draw on bodily resources for the most urgent repairs (fat plus muscle consumption might be enough to regrow just the head), while other healing can wait for more resources.

Or, more direct self-cannibalism might be used - pulling bone and muscle from elsewhere in the body whole to rebuild the head with, instead of just consuming tissue for energy/re-budding from scratch. I'm thinking tall and hunched means there's some "extra" spine and back that can be utilized - after all, what will it matter if they're 'straighter' or shorter until they can eat enough to bulk up again. Or they might have a bodily storage on top of their shoulders (rather than all of it being extra curved spine) - something like a camel's hump, and containing fat stores for energy, or raw materials for healing head injuries. I'm also imagining the head being crudely "re-purposed" from the shoulder or hunched-over bits of spine (folding up and over like an inchworm, and leaving the original much shorter), since that will be 'cheaper' than budding off a new head, and those kinds of repairs (height, or arm) can wait for extra resources.

Or maybe this mechanism isn't limited to just the head, but in general - a kind of low-level instinctive ability to repurpose their (adjacent!) tissues for patchwork closing wounds when resources or time for proper healing is scarce, could help explain the super-regeneration. They would slowly get shorter or smaller as they lost mass, and "re-purpose" what's left as best they can into a smaller whole body instead of a bigger one missing parts, and restore the mass (and height) with food energy as available. A few hundred leaps in evolution and some serious training down the road might get you limited cosmetic-level shape-shifting, but at this point it's an automatic instinctive healing process, and also possibly buggy as heck.

Ok and moving on, what about, 'can only be killed by cutting out the heart' - I'm thinking also no. Not only are there too many ways to be injured, healing itself is expensive as anything. I would expect starvation to kill off a troll relatively quickly, since it needs resources to heal and maintain itself. Significant levels of trauma will also kill a troll, since they would have to keep healing - stuff like being dismembered or continual pile-up of littler injuries until they kinda starve to death expending energy to heal. I expect vulnerability to temp shifts, since their metabolism has to be tailored to the regeneration - extra work from heating or cooling could be very bad. Getting the brain (well, brain stem) will also be fatal - though it might be hidden, or armored, or the attacker doesn't know to target it specifically (and minor incidental injuries can be healed). So cutting out the heart can be the most practical method for killing one in combat, even if it isn't the only way.

What else... Your regeneration is kinda insane, but you already knew that. Between the immediate clotting, reasonable patchwork, and the longer-term regenerative behavior, this is a biologically expensive system. There should be lots of drawbacks - especially if they're also big, tough and resilient (to resist injuries), and also powerful and dangerous (to prevent injuries from existing), as usually those big, powerful, dangerous use those traits to avoid being injured so much. Usually those kind of traits trade off for the dealing with the same problem (most creatures specialize rather than pay for all costs), so they need a reason to still need all that regenerating. Like maybe a really powerful predator, or dangerous environment, which they cannot match with force of arms or innovation alone. Doesn't matter what it is, just think of a reason nothing's come along with a better trade-off to the problems this regeneration system is gonna fix.

As for your catapult-orcs, this is a nasty business. Injuries will be frequent, huge, and crippling, and quick-regen doesn't cut it as a primary method of compensating for that (though it works excellently as an all purpose backup for everything that could go wrong). Does it help if the catapults have slightly better aim, or the internal layout of the cities crowded enough, that most orcs could expect to land on roofs and jump down separately? The shorter fall, and/or having their falls broken by thatched roofs collapsing under them, could reduce injuries quite a bit. Could they figure out (or the stolen plans include) some kind of catapult-capsule for traveling in? Probably originally for catapulting supplies or messages, or maybe just to make firing irregularly shaped orcs easier, that just happens to also absorb impact by crumpling to let them remain in useful condition. Just lashing them to a crude wooden or wicker framework and firing would help - the wood can snap and shatter apart when it hits, slowing the falling orc enough to survive (it can hack out of the remaining framework afterwards), and the regeneration deals with any non-prevented wounds. Luxury models might be stuffed with straw for actual cushioning.

Note - you've got them hunched over quite a bit, and I've leaned on that as well, but there's no reason for my adaptions to be as recent as being driven underground. Is there a reason they couldn't be hunched before (and use the hunch for extra storage, misdirection, and/or regeneration reasons)? That might be why they stayed tall instead of shrinking down to size, since they already had a coping method in place. Also the above ground peoples might not have remembered the hunch, or mixed up "height while extended" and "height while usually walking" while the orcs were out of sight, or just assumed the hunch was from living underground instead of a pre-existing trait.

Ok, I can't think of anything else. I hope this helps.

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  • $\begingroup$ Reading through this I feel I already covered up quite some parts of it. Brain tissue in the torso controls breathing & heartbeat; Memory cells in spinal chord remember basic bodily functions and personality traits; most urgent repairs now, rest later, up to some years to fully recover the brain; valves for breathing and swallowing were also topic already but I'm still not certain as to how the food gets there; no technology to reduce impact damage as they're not smart; They possibly did shrink but there hasn't been enough time for adaptation meaning they will shrink more in coming generations $\endgroup$ – Otto Abnormalverbraucher Sep 10 '16 at 23:24
  • $\begingroup$ I do like the idea of the regrowth being a very self-destructive process just to keep the body alive. There have been powerful races and predators in ancient times so I don't see trouble there. What's more important, the tremendous change of environment could have them require such an all-purpose means of survival. Consuming their bones, muscles and tissue makes me wonder whether there's a possibility of specialized cells to re-generalize, reforming into a stem cell. That'd possibly be a less energy consuming healing process, as the body doesn't have to create the cells. $\endgroup$ – Otto Abnormalverbraucher Sep 10 '16 at 23:33
  • $\begingroup$ You're right about them being possibly killed by damage other than cutting the heart out. Suffocation won't work as the deadly part is lack of oxygen in the brain and the organs stop working in result. Organs keep working controlled by torso brain cells, damage in the brain can be repaired. You can prevent their healing process by cutting off their nutrition input and there are other ways but in a battle, it's the best bet and the only way people know of. $\endgroup$ – Otto Abnormalverbraucher Sep 10 '16 at 23:39

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