Hydra regeneration explained with science
The Hydra is a mythical creature with the ability the regrow severed limbs, including the head(s). According to the dungeon and dragon 3.5 version of the hydra : each time one limb is separated from the body, it grows back. Sometimes, two heads will grow to replace the lost one, until it reaches a certain limit. The only way to kill the beast is either to cut all the heads or to kill the body. It is not possible to kill the hydra just by cutting the same head several times in a row.
Other species can also regenerate limbs, like the salamander. But the regeneration is somewhat different for the hydra since it also regenerate the heads as long as there is still one standing.
So, I have three questions related to the regenerative ability:
- Is it scientifically possible for the hydra to regenerate her lost heads? I'm asking because it is more complicated to replicate the head than it is for an arm or a leg. This is assuming that they have some kind of brain (in the head) but have the same intelligence as most animals.
- How long would it take to regenerate one head if it was as big as in the picture above?
- This is related to the above question: Considering the time and energy required to grow a head, could they really grow several heads in a row?
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1 answer
An article recently posted to Cracked, "5 Fantasy Beasts That Wouldn't Work (According to Science)" by Justin Crockett and Nathan Murphy, points out that a hydra might collapse under the weight of its heads:
The hydra can have as many heads as it wants, but it still only has one body, and it can only support so much weight. You try sprouting 20 more heads and then fighting a muscle-bound dude with a sword.
This means you'll have to make the heads light and not overly slow to regenerate. That got me thinking: what could look like a head but not actually be a head? (Consider the hind flippers of a seal: they perform the function of a fish's tail but are instead homologous to its ventral fins.) You could make the "heads" out of arms.
- Limbs would regenerate if amputated, as you pointed out for a salamander. Regeneration without magic is slow, but stories can be paced with other reasons why predatory pests (such as your heroes) might only occasionally being able to score an amputation.
- Limbs would have enough nerves to process reflexes. A newly generated limb would need some rest time to build up "muscle memory".
- Fangs would be analogous to teeth but adapted from claws, grinding things before feeding them to the smaller true head.
- The features analogous to eyes would be adapted from other claws. There are real skin conditions that cause skin to be light-sensitive. If a claw is adapted into a primitive lens over photoreceptor cells, reflexes could make the hand behave as a head that can "see" a predator.
- Cutting off the true head will kill it. But that's far easier said than done, especially with all those arms protecting the head. So "having to cut off all the heads" is more of a practical matter of cutting off all the arms before the head can be reached.
The head/arm analogy brings up interesting concepts for how it fights off random predators carrying a sword. One "head" could grab the pest and hold it, and then another "head" grabs the other end and rips it apart.
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