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Why this question is different from existing questions...

The closest question to mine is Anatomically Correct Cerberus. It is clear from the answers to that question and indeed it's axiomatic to the scientific community at large that polycephaly is not a particularly useful trait in OUR world. Therein lies the distinction -- in my post, I explicitly concede that point. My question is different because I'm flipping the script. Polycephaly is a losing proposition here on Earth, so let's envision an alien world where polycephaly makes sense.

In the Anatomically Correct Cerberus post, Green said: "I can't think of a set of circumstances where it's a clear and long-standing advantage to have three heads instead of just one." He has a point, it's a challenge. I read through a lot of material, and I just have some modest ideas. Still, just because some of us cannot think of the circumstances doesn't mean that none of us can.

I mean it's easy to say "it's just impossible, case closed." However, the most fascinating part of the topic (to me at least) is unresolved. Namely, thinking of a world with the circumstances to allow for polycephaly to occur is unresolved. That's why I think it's worth taking a deeper look. I would like to keep the use of faerie dust to a minimum (hence the science-based tags), but if it's a bit of a tall order for people, I can allow certain latitudes where necessary to generate useful and creative discussion.

Polycephaly

Premise

Having multiple heads, or Polycephaly is a very visually stirring trait that we often find in mythological creatures. Chimera and Cerberus are two such examples.

Cerberus

Conventionally, the polycephalous trait tends not to be favored by natural selection. It is a trait that is often very rare and even when it is observed, the organism is thought to be handicapped by the trait. Consider the following comments regarding a two-headed snake:

"Just watching them feed, often fighting over which head will swallow the prey, shows that feeding takes a good deal of time, during which they would be highly vulnerable to predators. They also have a great deal of difficulty deciding which direction to go, and if they had to respond to an attack quickly they would just not be capable of it." -- National Geographic

As I researched the concept, I became more acutely aware of the evolutionary challenges that would inhibit the existence of a polycephalous creature. In terms of evolution, having one brain is an expensive component -- let alone several. The energy resources that would be required would pose a challenge for the organism to be able to thrive and reproduce. However, just because polycephalous creatures do not fare well in our world, does not mean that there couldn't be a fictional world in which polycephaly would be an advantageous trait and thus favored by natural selection.

Question

I would now like to open the floor to answers that can address the following question: What kind of world could have the conditions that would be conducive to the emergence (and perhaps dominance) of a polycephalous creature?

Further Clarification

  • a herbivorous or carnivorous creature is acceptable
  • all biomes are fair game (tropical, tundra, desert, etc)
  • the ecosystem is a complex variable and for the sake of simplicity you may assume other ecosystem constituents as necessary, just make sure you have a small list of assumptions for clarity
  • (optional) Ideally, I would like to keep the scope as is, but to avert undue subjectivity or being too broad, if you so desire, you may limit the scope of the question to our buddy Cerberus (pictured above)
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    $\begingroup$ Hey everybody, for what it's worth, the exact wording: "What kind of world could have the conditions that would be conducive to the emergence (and perhaps dominance)..." Here I think the key words are world and conditions. Suffice to say, not just any world will result in the emergence of such a creature. You may rephrase it if you like, I suppose. "a viable natural history" isn't bad, it's more concise than my original phrasing. Maybe "evolutionary narrative" could work too. We already have some great discussion, just wanted to close any loose ends. $\endgroup$ – Arash Howaida Feb 10 '18 at 14:28

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The heads are not identical. Each is specialized through adaptive radiation like Darwin's finches.

enter image description here

The heads may not be limited to types of food, but for any reason heads evolve: to set a pair of eyes further apart and higher, another head for hunting underwater, maybe one head just to attract a mate (that one wouldn't need to eat or think, just look attractive), and another head for communicating.

At birth the heads might not be differentiated.

enter image description here

Lobsters are born with matching claws, but as they grow older one becomes dominant and they develop into two completely different claws called a pincher and a crusher. The heads on your creature might be similar with one head naturally becoming dominant, and the lessor heads developing for secondary tasks (whatever they may be).

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    $\begingroup$ And two of the heads have tiny brains focused only on watching for predators and whatever they're specialized for. The head with the largest brain controls the body and is connected to the other two. This would save energy from supporting three equal size brains, and prevent confusion or infighting. $\endgroup$ – Dan Clarke Feb 8 '18 at 3:57
  • $\begingroup$ bulbapedia.bulbagarden.net/wiki/Girafarig_(Pok%C3%A9mon) $\endgroup$ – linuxhackerman Feb 9 '18 at 9:47
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    $\begingroup$ This answer reminds me of a character I know (essentially a hydra-gryphon) where each of the heads have different personalities. That could fit right in with this answer. $\endgroup$ – Draco18s Feb 9 '18 at 18:55
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They have false heads

Some species today have evolved false heads in order to protect their real heads from predators. some examples include: Thorny Devil or Long-banded Silverline

By KeresH - Own work, GFDL, https://commons.wikimedia.org/w/index.php?curid=3716812 By Vinayaraj - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=51096074
Over time, your species could have evolved to have more sophisticated 'false heads' that look more and more like a real head. They may even evolve to have sensory organs, or be able to move like a real head.

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Your environment could be extremely hostile to smaller creatures, resulting in various strategies to become large. You'll have a lot of creatures which have many offspring which grow extremely rapidly in the hopes that some survive, but other options are available.

Your multi-headed species engages in a type of symbiosis, where its offspring develop on its body with a shared circulatory and nervous system. It would have multiple heads which could watch in several directions alerting it to potential threats. It will always have a range of head sizes from a single large one ranging down to tiny developing heads. The larger heads would act more independently, trying to eat their own food and potentially disagreeing with the others. As a head reaches maturity it would develop a longer more muscular neck (actually the rest of its body developing) before gaining legs and separating from its parent. A newly separated creature would only have one head for a time before it developed offspring of its own.

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Not particularly knowledgeable on the subject, but octopi came to mind. They do not have multiple heads, but their tentacles have neural clusters that essentially make them able to act and think, relatively, independently - in a sense, they can act as 8 creatures, or coordinate into 1. Coupled with their sensory input from their legs (not eyes, but octopi move a lot by touch) one can make a parallel with a head (eyes - sensors, brain - neural cluster). Especially if the creature you're thinking of designing actually has their main cognitive tool somewhere else, and the heads act more like semi independent extensions to allow multitasking.

Possible uses: Eyes in the back of your head, near perfect echolocation, multiple limb coordination (which would require a shit-ton of motor neurons), redundancy (an environment which you are pretty likely to damage your head, literally).

Just note that having multiple brains is expensive. Which might mean that food is abundant for this animal but, maybe, this animal itself is a food for multiple predators (so energy efficiency is less relevant than perception / defense capabilities).

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    $\begingroup$ octopus comes from Greek word ὀκτώπους, hence its plural has nothing to do with Latin plural (deus -> dei). Just FYI. $\endgroup$ – alecail Feb 8 '18 at 10:22
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    $\begingroup$ @alecail a comment that would be more useful if it mentioned the correct pluralisation (either anglicised: "octopuses" or Greek "octopodes"). $\endgroup$ – Pahlavan Feb 8 '18 at 13:13
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    $\begingroup$ @alecail You should read english.stackexchange.com/a/138236/5699 $\endgroup$ – corsiKa Feb 8 '18 at 21:31
  • $\begingroup$ @corsiKa I remember reading that text, but didn't realise it was on here :) $\endgroup$ – Will Crawford Feb 10 '18 at 0:22
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    $\begingroup$ @Pahlavan +1 to octopodes! $\endgroup$ – RonJohn Feb 10 '18 at 11:45
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The various negatives related to polycephaly are primarily because of multiple conflicting directives issued by two brains sharing a single body. Place a single brain elsewhere in the body, say protected by a bony hump in the central torso, and you can have two or more heads without conflict.

Which is basically how you get the Pierson's Puppeteers.

From an evolutionary perspective multiple heads would seem to be a contra-survival trait. Niven worked around this for the puppeteers by framing it as a defensive advantage that allowed a herbivore to better triangulate lethal kicks against a predator that was following it. This was so extremely advantageous in the primitive setting of the Puppeteer home world that they became the dominant life form.

Another thing to overcome here is that sensory data like sight and hearing needs to be processed extremely quickly and so needs the shortest possible path to the relevant sections of the brain. Having your eyes on the ends of tentacles with several feet of nerves leading to the brain would be extremely inefficient and almost certainly an evolutionary disadvantage. The solution in this case might be to have a distributed nervous system instead of a central nervous system. Have processing for the visual inputs happen in the neural network of the head with data fed out from there to the 'brain' elements in the rest of the body. Of course there are many potential downsides to having a distributed nervous system as well, but let's ignore them for now.

The final option I have for you is an environment which lacks suitable evolutionary selection pressure to select against the disadvantageous multiple-brain scenario. Given that two brains cooperating together on a single problem may in fact produce more effective solutions, and that cooperative brains would ultimately be more successful than uncooperative, a mutation that produced polycephalic offspring could in fact result in a polycephalic species. The environment would have to be quite benign compared to our own with minimal predation and so on to allow the initial few generations to survive long enough for the species to find its own survival specialization.

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  • $\begingroup$ I think this is basically the right answer, so I'll confine what I have to say to comments here rather than posting my own answer. Here are some other reasons to have multiple heads, given an architecture where the brain is (mostly) not in the head: (1) redundancy - you can lose one and still be able to eat. $\endgroup$ – Nathaniel Feb 11 '18 at 10:32
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    $\begingroup$ (2) limbs - suppose this planet's equivalent of vertebrates managed to evolve with only two legs and one head. Now if a species needs to manipulate objects in any way (or have an extra weapon, etc.), it will need another limb. In this situation a mutation that duplicates the head will provide a selective advantage, and you would expect it to evolve into many species with several heads each, with the heads tending to be specialised for different tasks. $\endgroup$ – Nathaniel Feb 11 '18 at 10:35
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What about one head being diurnal, another nocturnal and another cathemeral?

While the nocturnal and diurnal heads take long turns, the cathemeral head would be active during important parts of the day, such as when hunting for food or finding shelter.

Each head could in turn wake the other when in a panic (a sudden flush of adrenaline would do that for most creatures anyway) and then all three could be active, but only rarely.

Couple this with extra features, such as different sensory organs (say the diurnal head focuses on eyes whilst the nocturnal head focuses on its nose) the three could work together even while they are all awake.

I imagine this could come from pressure to be awake constantly. Perhaps a dire shortage of food requiring constant travel, or constant predation. A polycephal creature would probably be a large creature to support the heads, so perhaps its natural predators are small and pack-hunters, meaning awareness and the ability to escape early pushed the need to keep so many heads?


Alternatively use a head like a hand!

I believe a key point that makes polycephaly a problem is that each head attempts to act independently. Solving this by including a sort of ultimate brain located in the middle of the others, with the heads having smaller brains or perhaps even giant nerve centres as opposed to brains, may make more sense.

Essentially, take the brain out of the head and stick it in the chest, then control each head together. This would require a complex brain (good thing it has so much space in there!) to be able to comprehend so many senses (three sets of independently moving eyes, ears and noses is a bit much!).

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  • $\begingroup$ Robert L. Forward's Saturn Rukh comes to mind. They are perpetual fliers, landing is not possible. Thus they have two brains (and two heads) that take turns keeping it aloft. (Now, how it could evolve is another matter... His worlds are interesting but his evolutionary biology wasn't up to his physics.) $\endgroup$ – Loren Pechtel Feb 10 '18 at 5:04
  • $\begingroup$ @LorenPechtel Some animals managed that by sleeping with half a brain at a time (en.wikipedia.org/wiki/Unihemispheric_slow-wave_sleep). Two heads are not needed! $\endgroup$ – KC Wong Aug 14 '18 at 5:37
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The creature could reproduce by budding. Either Sexually or Asexually. Instead of growing their young in an egg or a womb, they simply grow out of their parents.

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This is not applicable for a wide range of two-headed creatures because I can't imagine this as an ultimate survivor strategy but there was mention of tool use in finches in another answer, so technically, having two heads with a long neck and a type of mouth or beak that can grip well could be a substitute of having two arms with opposable thumbs. (In a creature that is unable to use its limbs in a similar way because they were just not designed for it or are in the long place/height.) I.e. make the species capable of advanced tool use.

For this to become a trait, a lot of favourable conditions need to be in place 1) to introduce this as a trait and not just a one-time coincidence of birth and 2) a safe environment for the new species in that awkward phase when they haven't yet worked out how to use the new trait well enough to compensate for its drawbacks and turn it into a net benefit.

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What constitutes a head? In alien and otherworldly anatomy the answer may be very different than that which is familiar to us.

The head and neck arrangement as we know it has always seemed to me somewhat maladaptive. With the brain, eyes, ears, nose and mouth all in the head any injury to it is catastrophic. A broken neck or a ruptured trachea or carotid artery are all quickly fatal for a wild animal and easily sustained. The whole assembly could be considered a single point of failure.

So it doesn't seem unreasonable that in a rerun of evolution a different body plan might win out. Perhaps a creature might have a head for eating that can double as a backup respiratory tract, but isn't intimately entwined. A head each for two (or more) eyes as a protection against blindness. They could even have localised visual cortices.

Does it even make sense to have the brain perched up there? Maybe it should be in the centre of the body. Then having multiple heads has no significant impact on neurology.

I think considering the disadvantages of single headed anatomy could help you find a lot of ideas. I hope this small selection helps.

** Ecology ** To put this in an ecological context imagine a savannah watering hole. All manner of creatures must visit it for hydration and many would be at each other's throats, if they have them.

In the real world a drinking animal must lower its entire head to drink, which places its eyes just above the water level. A low bush could conceal a predator.

But if the animal has its mouth on a dedicated extremity then it may lower this to the water while its eyes remain high, granting greater visibility. This is arguably an advantage enjoyed by elephants who use their trunks to transport water to their mouths; a nice precedent.

The eyes could each stand alone, or even four eyes in pairs for all around binocular vision. With its mouth in the water but eyes and nose in the air, the animal would be much harder to sneak up on.

But if this anatomy is established early in evolutionary history then predators could also benefit. Imagine a muscular neck and jaws with sharp teeth buried, vulture like, in a carcass while elevated elevated eyes remain alert to opportunists or its own predators. Out could much more readily turn that powerful maw upon a foe.

High above, fruit eating avian creatures have only eyes in their head. A large brain sits at their centre of mass and a mouth between the shoulders, ready to snap up berries when, perched, the creature uses its over developed eyelids to pick them.

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    $\begingroup$ Hi, welcome to Worldbuilding.SE. That's a good number of observations that you have listed. However, in our world almost all vertebrates have a neck+sticking-head structure. To address the question by the OP, you could add to your answer some features of a fictional world that would make your suggestions the norm. $\endgroup$ – NofP Feb 9 '18 at 13:23
  • $\begingroup$ @NofP how's that? $\endgroup$ – user1876058 Feb 9 '18 at 16:02
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    $\begingroup$ Nice! Do I get it correctly that you are thinking of an extremely probable prey-predator interaction for vertebrates, hence the evolutionary selection in favor of animals with more "sensors"? $\endgroup$ – NofP Feb 9 '18 at 16:05
  • $\begingroup$ @NofP Yes, it seemed like a useful line of reasoning. Thanks for suggesting that I make that addition. $\endgroup$ – user1876058 Feb 9 '18 at 16:16
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It IS possible but exceedingly IMPROBABLE

As a thought experiment how would these 3 heads on one organism evolve under the current rules of our world (but with fantastic circumstances)? In order to evolve multiple heads in the current science you really only have 3 possible (general) scenarios.

  1. The creature evolved a head then was slowly favored to grow a second head from scratch via selection, and so on. This means that the heads are not the same as they have evolved independently. They won't have the same kinds, size, type, function, etc of brain, or face, or anything. This would be more similar to a a creature with a sheep head and a dog head, no 2 or 3 dog heads.

  2. Somehow a creature mutated and was born with 2 heads right away and this was selected by the environment to persist. Having a complex system like a head and brain just randomly be duplicated is probably so unlikely as to again be considered just impossible. * The only reasonable way this might happen is if you have a fetal abnormality that results in a chimera of 2-3 different organisms fused together. But again for the sake of reproduction we have a problem; only 1 of them will be able to reproduce because an egg cell only accepts one sperm cell. So offspring will be the child of only one of these chimera heads.

    The mutation of chimeraism would have to be favored in general (even when it doesn't results in a fused chimera), and then fused chimeras would have to somehow out perform the same number of organisms as independent individual creatures for this crazy fluke of a mutation to persist at all.

  3. The creature somehow evolved 2-3 independent brains right from the start. Developing brains is costly so developing 3 similar or identical brains is extremely unlikely if alternatives (and there are many better ones) exists. 3 real heads with each its own brain... I can only think of sexual selection pushing this forward (like the peacock feathers) because it is otherwise so useless.

  4. Finally the best option, the heads are not real heads but are appendages. So in this case the animal would probably have something more like a centralized brain and the heads would just be another kind of appendage (like arms/tentacles), but shaped as heads and with some of the same functionality like being able to see and being an entrance for food. This is the most reasonable but is obviously a redefinition of the term 3 heads.

So if life follows the general rules ours follows; evolution takes place over a long time, where the best at surviving and reproducing are best represented, there is almost no scenario in which multiple heads in its literal sense would evolve.

To have multiple heads be a plausible evolutionary path in their full capacity (3 identical heads and brains) you have to throw out life as we know it. For example maybe a planet where life evolves in seconds rather years, or the method of selection is NOT survival and NOT reproduction.

Either way 3 heads is a fringe case even in the multiverse.


Expanding on why this would probably never happen anywhere:

As a biologist I would say that a plausible scenario in which natural selection would favor evolving multiple heads is probably so improbable as to be impossible in any form of life or biology that resembles our own (even in wildly alien environments). I will put to you first the argument about why this adaption is improbable in our current universe no matter where you are.

The reason this won't plausibly happen is because of the way evolution takes place which sets up basic constraints for the population. Some seemingly unnecessary things can evolve if they have a big payoff (like peacock feathers and reproduction), but multiple heads combines some of the biggest no no's in all of evolutionary science and this adaption has to somehow be better than the alternatives to evolve (there were no simpler or better adaptions that could have taken place?).

The problems in general are:

I) Having an extremely costly AND unnecessary feature. The problem here is that the brain is the most developmentally expensive part of an organism which will negatively impact not just its early development but its entire life by requiring constantly more resources to exist thereby reducing its survivability, chances of making it to adulthood, making it out of the womb, everything. This is true in any environment. So unless there is a massive pay off at the end there will actively be selection towards removing these heads if they ever came to exist.

II) Evolving 3 extremely complex systems is not parsimonious, nor is somehow maintaining all of them. If anything can do the same job but with fewer steps to get there or cheaper, that's probably what happened instead.

III) The heavy resource requirement accrued means the benefits have to be extreme which means the organism cannot adapt well to changes. This means it probably didn't survive environmental changes that reduced food or any of the huge list of things it needed to survive to adulthood. This also means the 3 headed creature didn't exist long enough to develop 3 fully functional heads in the first place.

IV) Many more reasons I unfortunately can't remember right now as I am rusty.

So the environment doesn't matter so much as the basic contraints of selection. So to realistically make this possible anywhere, the basic principles of life or evolution have to be different. If they are different, they are beyond our current human comprehension. That is the best answer.

If you want to use one of the above examples, you will want some place doused with radiation where the females for some reason LOVE the multi-headed males A LOT.

In our universe however, if you ever really wanted to have these things around, a mad scientist or crazy government organization would probably be your best bet.

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Conjoined twins can have multiple heads, so this isn't necessarily an issue on the same scale as trying to evolve wheels.

What you need is an environment where conjoined births are frequent, and a propensity to conjoined pregnancy is inherited, but the problems faced by conjoined twins on this planet (e.g. inability to coordinate movement effectively enough) aren't enough of an evolutionary disadvantage to matter.

Perhaps a degree of telepathy that means the twins can actually run at a sprint from an early age, and a need for 24 hour vigilance from some apex predator would be sufficient for the species to survive long enough. Those unlucky enough to be born without a twin get eaten the first time they sleep away from home, so only the conjoined twins survive until adulthood?

Not exactly what you were asking for, but is it close enough?

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    $\begingroup$ No--I think you solved the issue of how they evolve, though! Conjoined is too random, but what if it's due to a mutation and ends up conferring a survival advantage? Two-head conjoined becomes the new norm and would no longer be considered a defect. $\endgroup$ – Loren Pechtel Feb 10 '18 at 5:06
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I'll go on a different tack: One day, the release of a toxic bacterial colony (from an evil scientist's lab, no doubt) kills many in the community, but 10 pregnant females survive. of these, 6 give birth to two-headed children (either a straight double head, or a Siamese Twin with a single body and a semi-separated neck). As luck (i.e. evolution) would have it, the expressed genes for this bicephaly are next to the genes for immunity from this toxic bacteria. Within a few generations, only bicephalics survive.

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Lets start with the Octopus. As has been mentioned, it has neural clusters in it's tentacles that let them operate independently. Then we have a long necked creature that sends its head in to directly eat the prey instead of relying on suction cups to pull the prey out.

One drawback is that while its head is neck deep into a crevasse, it cannot see predators sneaking up on it. So, a mutation with an extra undeveloped head would be good for keeping watch and biting attackers while it extracts its main head.

This only becomes really useful if the two brains communicate. Otherwise, mini head wants to run away while feeding head is twisted around in the crevasse, anchoring it in position. So, mini head slowly becomes more dominant (not being eaten has more survival value than eating).

Another mutation leads to another eating head. Twice the chance of getting food. This requires more coordination. The neural cluster that coordinates activity between the heads gets more complex. Eventually, most "thinking" happens in the neural cluster that connects everything.

Other heads may appear due to the same mutation until some optimal number if heads is achieved (which may be different for different species).

At this point, the "main head" may lose it's dominance to the central cluster that connects the heads.

In different species, the heads may specialize.

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"Natural selection" and "evolution" are often used interchangeably, but in fact, these are two different concepts. Natural selection is actually a subsidiary process of evolution. Evolution deals with "what traits make it into the genome of a given species", whereas natural selection deals with "what traits REMAIN in the genome of a given species". If you don't particularly care how the polycephaly evolved in the first place (though barring weird science and/or some magical interference, depending upon your setting, the answer there is almost certainly "random mutation"), you only need to worry about the second question.

We already have examples from our world where many non-beneficial traits (or even actively negative ones) remain in the genome, because while people think of natural selection as the promotion of what works, it actually is much more commonly a reductive measure, meaning it provides for the elimination of that which does NOT work. Now, in our world, polycephaly is a pretty crippling disadvantage, usually. So for natural selection to favor your polycephalous species, polycephaly needs to either A) confer none of the penalties that it does in our world (or at least, not enough to make the individual organisms of this imaginary species unviable), or B) confer advantages that heavily outweigh them.

Off hand, I can think of only two circumstances that would promote this particular trait by means of natural selection, and they would probably need to exist in tandem for the trait not to be weeded out. First, energy conservation would need to be less of a worry for this creature (or for all creatures) in your world than in ours. Something like a creature capable of direct photosynthetic absorption of sunlight would be a good start.

Secondly, aside from the abundance of available energy, I would expect conditions in such a world to otherwise be exceptionally unfriendly to our life-form, such that the additional weaponry provided by extra jaws and/or extra sensory input and/or redundant neuro-sensory hardware was almost a necessary condition of survival. I realize that my answer here was more general, but hopefully it gives you some angles from which to start considering the problem.

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Guard duty: At all times at least one head could be awake (while some of the other heads slept). Such a creature could never be surprised by a stronger predator, and never let a weaker prey tip-toe past it. On the other hand, saying how a trait might be advantageous says nothing if it could have actually evolved. An animal which rolls on wheels would have an advantage in locomotion, but there doesn't seem to be any plausible evolutionary pathway to it, hence it never happened.

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From the perspective of it being a prey animal, here are some ideas I had:

1) The most obvious pressures for more heads are more eyes to look with, and more mouths to feed. You wouldn't need complex brains for this task, most of these functions are ones that could be done without much conscious effort. Other commenters went in better detail about this if that's the route you're going for, but I'll add that an interesting twist to this is a regenerative ability with the multiple heads if they're simple enough; the main head could be the only one unable to regenerate.

2) Multiple heads could also be redundant 'back up' features for a particularly hardy animal. Redundant organs already exist in earth creatures, so obviously, that's something that has some evolutionary pressure, albeit not precisely like we're describing.

3) Another could be making the creature look more fierce with more apparent ways to defend itself (two or more sets of jaws or points of articulation to attack with). Pretty straightforward, although this is under the assumption that other polycephalous, successful predators with a common (multiheaded) ancestor to this animal would put pressure on it to retain its many heads. Also, keeping in mind that evolution favors energy conservation, an animal with many heads to attack should have reduced forelimbs or something in that regard (something that could also be used to attack other animals. It would not need both many heads and attacking limbs. The most realistic creature for this, ironically, is the snake, or something akin to a polycephalous theropod)

4) Lastly, what I find more realistic is the purpose of making the creature appear like there's more than one of it there at once, like a false group/herd. To add on to that latter point, it can have Zebra like markings (dazzle camouflage) to heighten the illusion further, and/or vibrant eyespots to spook wary predators or rivals when someone gets close enough for the initial illusion to waver. The heads could be largely nonfunctional or have a simple function. Totally up to you. It would be most effective to at least allow them a range of motion, like a tail.

I'll add pressures that a predator would need later when I think of some. I didn't intend to give an official answer to this! My comment was just too long c:

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    $\begingroup$ "More mouths" could be a standalone answer. $\endgroup$ – wetcircuit Feb 8 '18 at 4:31
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    $\begingroup$ @wetcircuit You don't need to have more heads to have more mouths. A starfish has no definable head, neither does a clam, yet they have mouths. If I just said 'more heads', I wouldn't be answering the question. $\endgroup$ – Tardigreat Feb 8 '18 at 4:34
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    $\begingroup$ Agree, just realizing "more mouths" would be a good enough answer in most genres. LOL. $\endgroup$ – wetcircuit Feb 8 '18 at 4:38
  • $\begingroup$ @wetcircuit that's fair! If you really want to separate function in a curious, but very alien way, you could have a two headed creature that uses one head purely or mostly for sensory input (vision and hearing would be most effective), and the other head only have the purpose of attacking and rending food. You could have the sensory head be hypermobile, like a tentacle or owl neck for nearly 360 degree feedback. Funnily enough, I think this sensory head would require more brain power than the other, so one could consider it the 'main head'. $\endgroup$ – Tardigreat Feb 8 '18 at 4:47
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    $\begingroup$ I think the only way that one is realistic though is if it evolved from a multi-headed animal and just retrofitted one of the heads for just one function, or you could propose something like Snaiad's aliens where the two 'heads' are really mostly an illusion; one head is actually the creature's genitals, it just has a jaw like protective covering over it that it retrofitted for biting. Very weird description if you don't know what I'm talking about lol $\endgroup$ – Tardigreat Feb 8 '18 at 4:51
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There are some problems, none is really unsolveable, but all together tend to restrict this:

  1. One brain is more than enough, so a multiheaded species will probably not have its brain in its head (although dinosaurs having two brains is probably bullshit, a species COULD carry its brain somewhere else);
  2. Having your brain separated from main sensory organs (e.g. eyes) tends to complicate things, but there are insects with stalk eyes and similar that solve this problem;
  3. Having secondary digestive entrance is probably the worst problem; I don't really think there will ever be something that has more than one fully functional mouth attached to its "heads".

So don't make it have its mouth on the heads and brain contained in it - make it specialized limbs that come with eyes/ears/whatever and place the mouth somewhere on the main body.

That doesn't need those head/hands to not have teeth, or at least something like lobster claws to rip stuff apart instead of chewing.

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Symbiotic relationships seem like the only way this could happen to me. Let's just take a dog as an example. You're a parasite or something else that requires another organism to do well to survive. Your dog in the fight gets rejected by the pack or attached. Ordinarily your dog would die, but your dog has you and you can help it fight. For some reason, you decide that the best way to help your dog fight is to grow an offensive head on it and literally fight the other dogs. Perhaps this is all your capable of. Perhaps this is just what you do. For some reason this actually works - maybe humans think your a god - and suddenly something else is living with you and your dog while giving it another head. This sounds pretty dumb, but there are parasites that eat fish tongues and then replace them. This defensive appendage parasite idea is far fetched, but plausible.

This also doesn't need to happen with a parasite. Maybe this creatures young just stay on it for a long time and simply look like other heads due to some physical features. Perhaps your creatures are just really weird furry scorpions.

Alternatively, a parasite could do something that forces this trait in its host. This wouldn't be an advantage to the host, but it would/could be a massive advantage to the parasite itself.

When it comes right down to it there aren't a lot of benefits to having two heads. It looks pretty cool and that's about it. There's honestly not a benefit that could be produced by having multiple heads that I can think of that couldn't be produced in a less troublesome fashion by just having part of a head.

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If such a development happened, odds are the extra heads simply wouldn't last very long, evolutionarily speaking, as "heads". Consider the beastie in the image. If the extra heads are primarily used for sensory organs, then odds are they'd get pared down to, essentially, two extra arms/tentacles with the organs on the end (basically, the jaws and everything necessary for them disappear). If they're used primarily as weapons, manipulators, odds are they get reduced to two extra arms/tentacles with grasping claws on the end (everything except the jaws disappears).

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Our brains are in our head because worm's sense cells were near the front and quick reaction to stimulus was helpful. You need an environment that forces a worm to have sense organs on its head and tail.

What if your first animals developed in a continuous stream of water with various speeds. Side to side movement to switch to a faster or slower flow would be preferred. Side appendages for this might make it difficult to "see" both forwards and backwards. For these animals the front brain would "look" for food in slower moving flows while the hind brain would avoid predators approaching from behind in faster flows. They would both control movement but the hind brain would override the front brain.

After these animals leave the stream the two brain configuration should be preserved because being able to watch your own back is pretty helpful.
I think the reason we don't is because early animals found it easy to detect food and hard to detect predators. I think any environment where predator detection was easy for a 100 cell animal might create two brained animals.

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