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This is the second question in a series about my ongoing worldbuilding project, which seeks to explain the evolution and biology of various fantasy creatures. The first one was: Is petrifying vision plausible?

Now, I ask you to review a concept I developed myself and inform me of its plausibility.

The hydra is a well-known beast featured in Greek and Roman mythology - a serpentine, many-headed beast that lives in swamps. In my project, hydras are portrayed as amphibians that mate like Ceratiid anglerfish - males fuse themselves to the larger female's body and mate with her throughout her lifetime. Eventually, the circulatory systems fuse, and the hydra becomes a chimera.

The reason that Ceratiids to do this (It is thought) is because encounters between individuals are very rare, and mating opportunities must be exploited to the max. Similarly, my hydras are also naturally rare.

To give you an idea of the exact life cycle of a hydra, here's a rough summary:

  1. All the hydras in an area will be born at the same time due to the short period of the hydra's life where it is mature. This is for predator satiation - the predators of young hydras (Namely giants) are overwhelmed by a relative feast of prey, thus letting the majority go.

  2. The male is born and immediately moves to find a female, which may take weeks or months. During this time, he grows from the size of a salamander to that of a man.

  3. When he finds the female, he attaches himself to her body and begins to mate.

  4. Every time fertilization occurs, the female lays an egg. However, hydras are constantly moving, so eggs are often far between.

  5. Eventually, the female will have between 6 and 15 males attached to her. Hydras live only to mate, and she - and all her mates - will soon die.

How plausible is this premise? Is it possible that such a phenomenon would evolve in a creature of said anatomy, and are there any problems that it would cause if it did come about?

If you require any further details on these creatures, think it is unclear or too broad, or have spotted an error, please say so in the comments and I shall amend the question promptly. Also, if you do give it a VTC, please say why, as closing a question without giving criticism is wholly unhelpful.

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    $\begingroup$ Nitpick: consider changing the "Hydras live only to mate" bit - this will mean that the multi-headed monster will not eat, so it's much less fearsome... How about giving free-roaming males an "eating frenzy" behavior, so they can grow quickly and survive with their (possible) minimal anatomy until they attach to a female, and keep that trait while they are fused - so the hydra's heads are constantly gnashing and gnawing at anything similar to prey in their vicinity. $\endgroup$ – G0BLiN Jun 6 '18 at 11:27
  • $\begingroup$ @G0BLiN I more meant that once hydras had successfully mated several times, they have no evolutionary purpose to live longer. However, they have to eat to get to the stage where they can mate and die. Also, that's a great idea about the constant eating - that's what I was thinking when I said "They grow from the size of a salamander to that of a man", I forgot to include that detail. Thanks! $\endgroup$ – SealBoi Jun 6 '18 at 11:39
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/Hydras live only to mate/

Life stages that live only to mate (e.g. adult cicadas or mayflies) do not grow, because they do not eat. So your hydras would stay the size they were when they became adults / reproductively mature.

If your hydras are rare and males immediately start looking for a mate, they will find their sisters. You need to have them less rare, or some sort of group egg laying site (like salamanders) or some dispersal phase or your hydras will suffer from consanguinity.

Other organisms with parasitic / multiple males have a big size discrepancy. The females are much bigger because they contribute body mass / energy to eggs. The males contribute only sperm.

If you have a parasitic male that attaches for the long term that implies the female has a long term survival - like a female anglerfish. If your female does not eat she does not have a long term (unless perhaps she were truly immense - but still she will use up her reserves quickly). I do not see any selective advantage to the male, the female or the offspring from having two shortlived parents irrevocably joined for their short lifespan.


If you have a cicada like life stage that lives only to mate, you need a larval stage where the hydras accumulate energy and body mass. This could also be your solution for dispersal. The larval hydras could be aquatic, highly mobile and disperse widely. It is not until they metamorphose into the adult stage that they carry out the behaviors you describe. Some fish assume gender according to bodily resources - the big ones become female because they are big enough to do it, and the little ones are male because they do not have bodily resources to make eggs. So too your larval hydras - the rare lucky ones grow big and then become female.

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If they only live to mate, wouldn't it make more sense for them to protect and look after their eggs? And how can Giants feast on them if the eggs are spread wide and far apart. It feels like you want a creature with a turtle like egg laying pattern, rather than an Angler although you could just borrow the fertilization part. It would make more sense, that after the hydra accumulates 5-15 males, she rests and lays eggs until she dies. Her corpse becoming food for the new generation and the sudden boon of 1000's of baby hydras confusing any giants who are nearby.

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Temperature dependent sex determination.

This how you get one long-lived large monster with several heads.

Mummy Hydra lays eggs throughout the year. The Summer eggs hatches all females that feed and grow much like crocodiles. Most of them are picked off by predators early in life. Those that survive reach mating size in about five years at which point they emit pheremones to attract males.

The Winter eggs hatches all males. Males immediately begin searching for a nearby female to attach to. Most of them fail or are picked off by predators. Those that find a mate are ensured safety by the size and strength of their motherwife.

This way the adult symbiote monster can live a long time and grow to monstrous size through its lifetime. You also prevent siblings attaching to each other which is bad for genetic diversity.

Edit: Rather than laying throughout the year I suggest two large clutches in Summer and Winter respectively to 'overwhelm' predators as you suggested.

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