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I'm creating a species of humanoid creatures that live their entire life in couples.

From birth to death, they are bound emotionally, they work together as if they were only one individual but they are actually two, and need each other to survive. For some reason they can't survive if they are more than 20 meters afar from each other, after a few days they die.

So how does that work if we exclude the use of magic? Why would they die if they don't stay together? And why would such inconvenient feature evolve?

Edit: Since they are bound for life it is more logical that they are siblings

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    $\begingroup$ Since they are bound since birth, I guess they're siblings. Which leaves room for another couple for mating. Did you plan anything for the reproduction cycle ? $\endgroup$ – Keelhaul May 30 '17 at 10:23
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    $\begingroup$ So do they mate together? $\endgroup$ – Mormacil May 30 '17 at 10:27
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    $\begingroup$ Many plants and fungi have symbiotic relationships with each other to get food. I suspect food would be a good driving force here as we'll. $\endgroup$ – Bellerophon May 30 '17 at 10:27
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    $\begingroup$ Make sure this connection also provides some really good benefit, because otherwise in a realistic setting the strongest specimens (those who can resist more to the bond and can survive longer without their mate) would prevail and, over generations, the bond would disappear from their species due to survival of the fittest and evolution. $\endgroup$ – Hankrecords May 30 '17 at 10:58
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    $\begingroup$ Historically, people (and animals) grew up with a bunch of siblings and only a couple survived childhood. Using siblings would further reduce chances of survival $\endgroup$ – Raditz_35 May 30 '17 at 11:16
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I doubt it would've evolved in it's current form. I'd say it used to be a pretty useful set-up. Two small organisms working together. But as lived evolved more complex their dependency remained. Maybe they both specialized like how most life on Earth has two types of reproductive cells, male or female. One specializes in nutrition while the other specializes in seeking the other out. Now you could argue this isn't a fair split but that's not my point.

Perhaps they share certain bacteria required for their bodies to function. These die after a few days so need to be replaced. When they were still very primitive perhaps each specialized in different types of food. They then swap the broken down elements.

An advantage of having another member of your species around is genetic diversity. Sure mating with direct siblings isn't the best for diversity but not unheard off. Having a more diverse sibling might even offset some of the negatives. Still you might want to opt for mating with another pair. Maybe they're solitary and only meet other pairs during their mating season?

Humans aren't singular creatures either. We're made up of many different bacteria, without them we become ill or even die. Only in the recent years we've been aware of the importance of healthy gut bacteria. So having some form of a bigger dependency isn't that far fetched. For how it originated I'd look at breaking down food into nutrients or the immune system.

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This sounds like imprinting. Usually this happens a critical phase in the life of an organism. This could happen with siblings especially if the imprinting occurred shortly after birth. However, this doesn't prevent imprinting taking place after birth when an individual can be exposed to their bonded partner. This partner could be introduced by the parents of the individual, so they can both be pair bonded (this doesn't necessary need to be a sexual bonding, but it could be).

If the bonding is sexual this could involve the two individuals bonded together so once they reach sexual maturity as biological adults they can indulge in reproductive behaviour. Where the two bonded individuals are of the same sex, they will presumably engage in reproductive behaviour with another bonded pair of the opposite sex.

The proposed organisms that are bonded pairs are humanoid. This suggests that they will capable of complex behaviour and cognition, however, this wouldn't prevent their species capable of being imprinted.

This answer has mentioned that imprinting doesn't need to be exclusively at or immediately after birth. It can be done at other critical phases in the life the humanoids. The bonded pairs can be either of the same sex or the opposite sex.

Also, imprinting might vary during the lifespan of the humanoids. For example, siblings might bond together as a group. This could be for their protection or to make their lives easier. Later on as puberty approaches this sibling bond might break down or weaken, then the humanoids might be introduced to members of the opposite sex to imprint on each other to form a bonded pair who will then mate for life.

It can be assumed that imprinting of this kind will be produced by evolutionary factors in the environment where the humanoids arose.

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Many creatures in the real world cannot live without a partner creature. Examples such as lichens and mycorrhizae come to mind as well as the gut bacteria and fungi in animals, particularly cows which cannot easily digest food without them. These are all examples of mutual symbiosis, a relationship where both organisms gain. The difference between your scenario and these symbiotic relationships is that both your organisms are the same species making the situation different with its own challenges.

Even so, real world symbioses presents a good starting point. Most symbiotic relationships work by giving each organism one of two things: food or shelter.

Personally I would go food. Here the reason the individuals die if they leave their partner is because they cannot feed themselves. Where it becomes harder is working out why. In the real world it is because different species have different adaptions. Plants make sugars which fungi eat, fungi brwak down soil for nutrients plants need. Pretty clear case for symbiosis their. When both organisms have the same biology it is harder but not impossible. Most creatures have some form of louse or skin parasite. In these creatures the parasite could contain vital vitamins for the creatures meaning they have to eat them but eating parasites feeding of their own skin is dangerous, the parasites eat waste products and toxins from their blood which they cannot break down. These toxins are slightly different for every individual so eating someone else's parasites is ok. Unfortunately eating a strangers parasites allows disease to spread from person to person so the only safe parasites are those from someone you have been in such close contact to that you have the same diseases and immunities to them. Alternatively, if a pair are always one male and one female then it can be that males and females produce different toxins so males can only eat female parasites and vice versa then have pairs live in isolation so only have one another's parasites available.

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1) Make them addicted to each other via pheromones.

2) Have one produce vital substance A and the other one substance B. They have to share some kind of body fluid or something in order to survive.

1-2 can in theory be avoided using technology, e.g. pheromones in a bottle.

3) Make them psychologically so weak that they die without contact to each other.

Would it evolve? Well, you can never say never, but there are several complications which should be obvious. The same can however be accomplished otherwise. Make the families have an arrangement that before or shortly after birth a bond is made (which could lead to interesting political constructs in your stories) - kind of like arranged marriage. Then when you have decided why they would die, make a ceremony according to that - which would require the species to be somewhat intelligent. Let's go through the suggestions again:

1) Get a high dose from their pheromone glands and expose the other baby to it. This should get them hooked.

2) Same as 1), but here another way is possible: Have the adults cut out a part from the body of the babies that would produce A or B. This of course doesn't make the individuals partners unique, but it would be unlikely to find another fitting combination of 2 specimen - so the pairing is pretty unique. This only works if this is possible with your species, also there should be a lot of options of what part can be removed. Also this wouldn't work if there are billions of those individuals but only if there are relatively few of them.

3) Well, just raise the hell out of them with (religious?) terror and all of that.

EDIT: After thinking it through again (see my comment on the initial question), I am more convinced that a cultural rather than a biological path should be taken because of high mortality rates. If you choose option 2) or 3) (or others, one could easily come up with more) and one of the kids dies early - which is super likely historically - the partner could be replaced. For 1), this is somewhat more difficult. Maybe it would work with a specimen from the same family for example or it isn't lethal if the creatures are still young enough.

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    $\begingroup$ I'm voting number 3, emotional scarring is often most effective $\endgroup$ – Fl.pf. May 30 '17 at 10:54
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    $\begingroup$ Law enforcement could work just as fine as a cause of death, too. $\endgroup$ – John Dvorak May 30 '17 at 12:47
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    $\begingroup$ @JanDvorak Good point, this would require a highly organized society though. There are multiple ways of doing this. My main aim was to suggest an alternative approach to fantasy biology to make it more interesting and believable. Feel free to add more and more $\endgroup$ – Raditz_35 May 30 '17 at 12:53
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    $\begingroup$ "They have to share some kind of body fluid or something". Dude, they're like, related... $\endgroup$ – Grimm The Opiner May 30 '17 at 14:18
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    $\begingroup$ @GrimmTheOpiner Where you see indecent behaviour I see another species with their own customs. Check out cats and dogs, they will share some fluids with each other and their owner on a non-sexual level. And I'm sorry, your very own mother has most likely shared a couple of body fluids with you - and worse - this is how you get a gut fauna. I also dislike the siblings idea $\endgroup$ – Raditz_35 May 30 '17 at 14:22
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There are examples of colonies of unicellular beings acting as a pluricellular one, with each individual being highly specialized. like the Portuguese man o' war

Despite its outward appearance, the Portuguese man o' war is not a jellyfish but a siphonophore, which, unlike jellyfish, is not actually a single multicellular organism, but a colonial organism made up of specialized individual animals called zooids or polyps. These polyps are attached to one another and physiologically integrated to the extent that they are unable to survive independently, and therefore have to work together and function like a so-called individual animal.

You may need some tweaking to make it humanoid, but in principle it is possible: for example one could ensure motion, the other methabolic functions (digestion, excretion and so on).

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Dogs suffer from separation anxiety. This can lead to some quite seriously damaging behaviour in extreme case. Usually it's distress/howling/causing trouble... but there's cases where dogs have driven themselves to self harm as a result of that anxiety.

My dog suffers to quite an extent - she's extremely distressed when we leave her behind. This has lead to her 'breaking out' of the house - getting up on the counter top and pushing the (latched) window open, before climbing out, and trying to dig her way through doors to 'get through'. So whilst we are working on the issue, she doesn't get left alone.

Humans have similarly been known to suffer the same - usually we can adapt to it - but Separation Anxiety Disorder is a recognised thing

So I think for your setting - taking this, and amplifying it's seriousness would be sufficient to achieve your goal. Extreme anxiety is seriously debilitating. It's not directly fatal, but suicide and risk taking can be coupled with it, and it can significantly impact health and wellbeing.

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