Why do trolls regenerate?

For a while I’ve wondered why everyone agree that trolls can heal their wounds by regenerating. As a monster type trolls are recognizable by being big, strong and for the most part not particularly bright. Depending on the setting for a given book or game, the trolls may look or behave slightly differently, but they all agree on a single trait: regeneration. This spans different settings, whether it is Dungeons and Dragons, the Might and Magic-series or a dozen other examples. In fact D&D feels so strongly about this point that fire or acid damage is required to finish them off.

So where did this idea originate from and how come it is so widespread? It’s not mentioned in the old fairy tales, at least not any I can remember. The closest would be the one about the troll who hid his heart someplace else and thus couldn’t be killed. They’re tough? Yes. Hard to kill? Certainly. Often have multiple heads? Ok, most games have glossed over that aspect for some reason. The core attributes are the same, making trolls challenging opponents in whichever form they take. Adding the ability to close the wounds and restore it’s health over time, they’re also able to make a comeback even when you thought the fight was over.

I did some digging and part of the answer is probably that Dungeons and Dragons included it. Most, if not all, computer role playing game are influenced by what D&D created, so it only makes sense that it would spread to other settings. So where did they pick it up? Turns out there’s a book named Three Hearts and Three Lions by Poul Anderson which feature regenerating trolls. This seems to be where D&D got the inspiration. The same novel also contains the basis for the aligment system and the paladin class! I don’t know if the story explains why or how trolls gained this trait, and I didn’t find any more details. Still, looks like Three Hearts and Three Lions was the original source, and that after it got added to D&D it spread further from there.