The Hydra is a Greek mythological creature known for its many head that only increase in number as you fight it. Alcaeus was the one to solidly state the the creature started with 9 heads, a large primary head and 8 smaller secondary heads. Along with the magical details previously stated it was also the size of Hercules himself, over 6ft.

  • Is there a realistic way that a Hydra could evolve?
  • Using earth or near earth biology how close could I get to the Hydra?

An answer that can realistically explain the hydras regeneration, along with its size and head count will be accepted.

A list of all of the Anatomically Correct questions can be found here

Anatomically Correct Series

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    $\begingroup$ There is already a question about the regeneration: worldbuilding.stackexchange.com/questions/10054/… $\endgroup$ – Vincent May 21 '16 at 23:35
  • $\begingroup$ @Vincent but the question asks for more than just the regneration $\endgroup$ – TrEs-2b May 21 '16 at 23:46
  • $\begingroup$ @TrEs-2b "An answer that can realistically explain the hydras regeneration, along with its size and head count will be accepted." Either this is a duplicate, or you'll need to tweak your question. $\endgroup$ – Schwern May 22 '16 at 0:32
  • $\begingroup$ @Schwern it is not a duplicate as it does not ask for the regeneration aspect of the Hydra alone, but instead asks about the hydra as a package deal. $\endgroup$ – TrEs-2b May 22 '16 at 0:52
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    $\begingroup$ @TrEs-2b Since regeneration was covered does that knock it down to "why would natural selection produce a 6ft tall beast with 8 secondary heads?" $\endgroup$ – Schwern May 23 '16 at 2:21

There's a good reason why animals don't have more than one head. Heads use a lot of energy, and exist because keeping the sensory organs in one place (the direction of the creature's motion) is an efficient body plan - more than one head would not benefit the organism much and would only cause problems. Regeneration is also very uncommon in animals with a complex, central nervous system, since nervous systems tend to develop in a very structured manner and generally need to start from scratch to form properly. Even vertebrates that can regenerate (like some lizards) cannot regenerate their heads.

While a creature with many heads is extremely unlikely to evolve and one that can regenerate their heads is even more unlikely, there are some possible ways you could come up with something that looks and acts like a Hydra without actually being one.

  1. A descendant of octopus that has become more cephalized, developing a proper head and a long neck, as well as adapting its tentacles to look like eight 'false' heads to distract predators. Some caterpillars and fish use similar strategies, with eyespots on their back ends so predators attack the wrong part. Octopus tentacles are already capable of regenerating, and sometimes a severed tentacle will regenerate two new ones if something goes wrong in the regeneration process. This fits the original legend quite well.
  2. A social species of snakes that live together in a single nest. They might join together for protection or to take down larger prey, which means they would probably have more 'tearing' teeth to share kills, as opposed to modern snakes which swallow their prey whole. They would not actually regenerate, but as more snakes leave or join the colony (or hide in the middle) the number of 'heads' would seem to vary.

Make the hydra live in an area where it has to probe crevices and cracks for food. It has muscular tentacles that each have rudimentary sensory organs and a feeding orifice. So it looks like a beast with multiple heads but they are really just very specialized limbs designed to probe into cracks, identify food via crude vision, motion detection, infrared, etc, and eat it. Obviously it would be very difficult to regenerate additional limbs within the timeframe of a single encounter but perhaps it can extrude pseudopods or mock limbs when threatened to look more imposing, even if these pseudopods don't have the same sensory ability and eating orifice of a regular limb. The limb is more like an elephant trunk that can bite and tear, swallowing food via peristaltic action. It can color it's skin like an octopus so when it extrudes a muscular pseudopod it can color it to look like a regular limb.

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    $\begingroup$ That's what I thought when I pondered about Hydras. There's a single brain in the body(probably one and the same with the spinal cord) and outer extremeties with sensory organs and rudimentary nerve system organs(lesser brains) to operate them. The loss of the extremety is not fatal and it might regenerate over time. All in all it's a whole different, probably a parallel genus to the vertebrates. $\endgroup$ – Nick Dzink Jun 24 '17 at 13:12

No clue for the realistic regeneration (I think there's got to be a lot of handwaiving involved, especially for the regeneration speed that produces an entire head within minutes). But you can get a multi-headed creature through birth defects.

So, basically your Hydra is a long-necked lizard/dinosaur creature. Size-wise, you've got your choice since dinosaur size could cover anything you want from one-foot tall all the way up to ten-foot shoulder height. And when you've got the embryonic cell division faulty, you could get siamese hydra-twins. Sharing a body, but having multiple heads/necks. It is very possible that the Hydra species has a genetic predisposition for such siamese birth defects, with the conjoined-body - separate-head one most prevalent.

Of course, a nine-headed hydra would then be a real miracle where you will probably need to make use of your enhanced Hydra-regeneration to make such a disposition survivable (I don't want to think about the additional strain on the heart to pump blood not up one long neck only, but 9 necks including 9 very resource-hungry brains). And, as a side-effect, you'd have all kinds of Hydra head-counts, from one or two heads (most of the Hydra population) all the way up to 9 (or more, if you are feeling inventive and generous with the regeneration boost). And one-head-eight-legs hydras. And Hydras with multiple tails or eyes or multiple anything.

So the question remains why such a siamese-twin-prone species didn't get erased by evolution before they even started to become a species? Well, regeneration might be the key factor here. If it is coupled with the siamese-twin-propensity, that would be a deciding factor -- regeneration enhances its biological fitness so much that it offsets the siamese twin defect that comes with the regeneration boost. Since the super-regeneration must have something to do with enhanced cell division, it might already kick in in an embryo and cause all kinds of trouble there -- most notably preventing a full twin from forming (splitting of the embryo) and instead growing back together even before the split was complete. Ergo: siamese twin hydra.


Automimicry is when a body part looks like another, usually more vital, body part to fool predators. For example, there is a lizard in central Africa which has a tail which looks like a head. Many gastropods have retractable antennae which can be hoisted back into the body or extended outwards. Place these ideas into your magic box of Evolution Does Not Really Work Like That and bake for a few million years so that it grows to 6 feet on land and develops many automimic eye stalks.

It can't regenerate heads, but it can extend spare headlike appendages at will, at least until it runs out.


There actually is a whole phylum of Platyhelminthes, species of which (mainly from families Dugesiidae and Planariidae) can have a lot of independent, fully functional heads.

For example take an Dugesia polychroa - you can cut their head axially in half into like Y shape of the animal. Then both of the head-halves will regenerate into fully functional heads. Yes, you can repeat and repeat for as many heads as you wish.

This works also with the whole animal, so you can cut it into multiple parts and all of them (if not too small) can fully regenerate.

Scaling this up to the humanoid form is actually a minor issue.

Im creating a map of of their populations on my web http://brmlab.s0c4.net/bioosm/, if you click on the Platyhelminthes button, you will see... :)

  • $\begingroup$ Hi @sachy, welcome to Worldbuilding.SE! We hope you enjoy your stay, make sure to check out our help center and take our tour if you haven't already! For your answer, do you mean Platyhelminthes? I'm editing to correct the spelling $\endgroup$ – JSCoder says Reinstate Monica Aug 29 '18 at 14:06
  • $\begingroup$ "Scaling this up to the humanoid form is actually a minor issue." If it were really so minor, we'd see life more complex than flatworms with this ability. $\endgroup$ – RonJohn Aug 29 '18 at 14:13
  • $\begingroup$ @RonJohn I mean "minor issue" in the context of worldbuilding and this Anatomically Correct series. $\endgroup$ – sachy Aug 29 '18 at 15:44

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