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What sort of circumstances would allow for a parasitic relationship between two dominant species of an Earth-like planet? The criteria for an answer is as follows:

  • The relationship must involve no more nor less than 2 separate species
  • Both species must be dominant, ie. independent, intelligent and capable of recognition of the existence of the relationship
  • The relationship must benefit one of species whilst harming the other
  • The relationship must be evident throughout the species, not only in a select sample
  • The relationship must be ecologic or biologic, ie. a war between two dominant species, where one species is more advanced and clearly will win, does not qualify

Criteria aside, the question is how would such a relationship naturally develop whilst both species, especially the harmed species, remain dominant. The second point states that both species must be aware of the relationship; if this is not possible, this statement may be omitted.

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    $\begingroup$ I am stretching definitions a bit, but the social dynamics of dating (between men and women) in the western culture satisfy many of your requirements. It can also be interpreted as "A provides a scarce resource that B craves" as in @AdamPhelps's answer. $\endgroup$ – Federico Poloni Jan 25 '15 at 10:25
  • $\begingroup$ @FedericoPoloni awesome comment :-D $\endgroup$ – Sheraff Jan 25 '15 at 10:47
  • $\begingroup$ @FedericoPoloni I am annoyed that I didn't get notified of this great comment. Truly brilliant. If you could accept comments, I would :D $\endgroup$ – blaizor Jan 25 '15 at 11:10
  • $\begingroup$ @FedericoPoloni Would you agree that abusive relationships would qualify slightly better? $\endgroup$ – SBoss Jan 26 '15 at 16:45
  • $\begingroup$ Dating usually benefits and is consented to by both parties. $\endgroup$ – Donald Hobson Jun 17 '16 at 18:04
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I don't think you have to go outside of Earth's history to find a situation close enough to your criteria that minor modifications wouldn't satisfy. For example take one population being supplied with alcohol from the second population.

More explicitly, we have group Alpha and group Beta, each dominant civilizations of differing phylogenies. Secondly, we have a resource highly sought after, but destructive to, Beta. Alpha has a constant supply of this resource that Beta can't access on its own for whatever reason. Alpha could make whatever demands they want for the supply of this and Beta would likely comply within reason. It could be a fairly stable relationship on the order of centuries or millennia. Beta would likely eventually evolve out of a situation like this, but the volatility of civilization and politics probably means that this wouldn't be the constraining factor (in terms of time).

A few possible examples of the resource:

  • Alpha secretes nectar similar to aphid's honeydew that gets Beta high.

  • Alpha can create a light show similar to cuttlefish that puts Beta into a sort of trance that they love.

  • Alpha and Beta are closely enough related that there is significant overlap in terms of sexual desire but reproduction is impossible or offspring is sterile (eg. horses and donkeys). Beta prefers Alpha mates over Beta mates (donkeys prefer horses but horses don't like donkeys). Alpha supplies Beta with mates or prostitution, significantly retarding Beta's reproductive rate.

Some more examples that don't fit the bill for "resource" but could still plug into the equation:

  • Alpha has clear, strong mathematical intuition, Beta does not. Alpha hustles Beta in gambles.
  • Alpha is persuasive and Beta is gullible. Alpha constantly talks Beta into doing Alpha's bidding and Beta only figures out later what happened.
  • Alpha has properties Beta had long considered properties of their deity. The lay Betas are always worshiping Alpha despite the educated Betas knowing better.
  • Beta is altruistic. Alpha is opportunistic.
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    $\begingroup$ This is my first answer on this site. Please let me know if I can do anything to improve it. $\endgroup$ – Adam Phelps Jan 25 '15 at 8:07
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    $\begingroup$ It's a good answer. Welcome :-) Don't hesitate to add to it as the question gets refined or the discussion advances. $\endgroup$ – Sheraff Jan 25 '15 at 9:31
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    $\begingroup$ A great take on what is for most people a difficult concept to grasp, and even more difficult to communicate through a clean, concise and complete answer. Accepted, upvoted $\endgroup$ – blaizor Jan 25 '15 at 10:08
  • $\begingroup$ Great first answer. Welcome to the site! $\endgroup$ – a CVn Jan 26 '15 at 8:50
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I think about humans and rats, which found a niche in human agriculture and followed us everywhere. Why are rats, roaches, and other pests dominant (living off our success) rather than killed off? Not for lack of trying!

So the only difference you want is sentience. Would it still work if rats or roaches were intelligent?

A big issue, I think, is the size difference. If another life form was similar in size we'd call it war. Pests need to find rich environments in our incidental infrastructure.

It is reasonable that being clever is a benefit, to keep ahead of the host. If the host is intelligent so can eliminate niches, create traps, and otherwise counteract the parasite using brain power which is much faster than biological evolution.

Pests solve that in real life by evolving rapidly (germs) or fast enough (roaches), but for non-microbes a main property is hiding so any attempt to kill the population will miss some; and fecundity so will bounce back if only a few are left.

If fecundity was strongly selected against, cleverness might take its place. Rats are already smart in some ways, with social behavior and learning, but the layout of the brain is the Mammal, same as ours but much smaller.

A bird has a different brain structure and some birds are surprisingly intelligent for such a small size, and smarter than previously thought. Point is, very unrelated species means the brain can be more different, not similar to ours but too small.

It's also noted that hive insects are like an extended organism and lots of fiction posits intelligent ant hills, when individual ants are not. That brings me back to cockroaches: a distributed many-body creature could be intelligent with a long life and low reproductive rate, while the component bodies are quickly replaced, tiny, and specialized. But that does not physically evolve as the units are produced by the fixed organism, not descendants with variation.

We might not realize that the pests are intelligent! But a hive-being could live off our productivity and co-evolve with us, the whole time being a pest. Our continuous attempts to wipe it out would only drive its evolution toward being a better pest.

An interesting story might be the discovery of the intelligent hive, and coping with its world-view.

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    $\begingroup$ A very valid, refined answer. This answer gives a lot of relevant information that I had considered, but not this deeply. I'm impressed, but this answer is more of a resource, reference or foundation for an answer that focuses on the intelligence and dominance of a species. What I meant by dominant was a species similar to humans in the sense of a large population capable of communication and what not. I will upvote your answer regardless, as it has enhanced my understanding of the accepted answer. $\endgroup$ – blaizor Jan 25 '15 at 10:17
  • $\begingroup$ I was writing my comment already! Your answer is good but it has a lot of typos and no structure which make it very hard to read. I'll remove my downvote when these are corrected. $\endgroup$ – Sheraff Jan 25 '15 at 10:27
  • $\begingroup$ Feel free to fix typos. I'm on a tablet which makes long answers a pain. $\endgroup$ – JDługosz Jan 25 '15 at 10:34
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    $\begingroup$ Sorry, but I have to agree with @Sheraff . I was so caught up in the content that I failed to observe quite a number of structural faults, along with a number of typing errors. Please review. I will not remove my upvote, as it is a good answer based off the content, but I suggest you review your structure and spelling. I hope you take this constructive criticism lightly, as that is how I intend it to be taken. $\endgroup$ – blaizor Jan 25 '15 at 10:36
  • $\begingroup$ @jdlugosz I submitted an edit that corrected your typos as you requested. I hope this helps. $\endgroup$ – Adam Phelps Jan 26 '15 at 5:47
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Leaving aside the sentience part, there are very few examples of accepted parasitic relationships in nature. The only one I can think of is how some trees deliver drugs to ants in exchange for them protecting the tree, like acacias do. I do think that sentience could be a deal changer in one of a few ways:

  • The host specie is extremely altruistic and decides to allow the parasitic specie to live off them out of a moral decision (I mean, some humans are already altruistic to the extreme, even to other species, like Grizzly-Man who got eaten by a Grizzly, probably because he didn't want to fight it).

  • The host specie is voluntarily blind to the harm done by the parasitic specie and prefers to live less long than to live without the parasite. In this case, the parasitic specie has to provide some addictive or high value material. We could think of drugs/alcool, socially enforced aesthetics, mental/physical enhancements... All the things humans are already ready to sacrifice a bit of their life for. This could also work the other way around, where the parasite allows for a longer life for its host, but with a lower quality of life.

I believe that even in these two cases, unless the parasitic specie as a control over the host that is unknown to the host, some (and maybe a lot of) hosts would refuse the parasites.

  • The host specie needs the parasitic specie (this could almost qualify as a symbiotic relationship, see below, except that here, the parasite still harms the host). You could imagine the parasite being the only solution against a very deadly disease for which no cure was found, or maybe food sources for the host specie are depleted and only the parasite is able to generate enough energy from other sources...

On the other hand, you could consider a mutual symbiotic relationship — which could still fit your narrative as some (more or less rare) combinations of host/symbiont could lead to harm to one or the other. Mutual symbiotic relationships can be separated into two main categories:

  • Endosymbiotic mutual relationships, where the symbiont lives inside (or attached to) the host. It is often the case of bacterias/pluri-cellular host, fungi/plant, virus/pluri-cellular host. The host usually provides mobility and protection, the symbiont provides a useful function (energy production, protection from some diseases...).

  • External symbiotic mutual relationships, where the host and the symbiont are separate but cannot (or won't) live without each other. Most common examples are flowers/bees, whale sharks/remoras, leafcutter ants/fungus, anemone/clown-fish... In these cases, the two species provide an array of useful functions to each other. This is the most equal relationship.

I believe that in these two cases, almost all of your host population would host a symbiont.

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    $\begingroup$ I originally had titled the question using symbiotic instead of parasitic, as I am extremely interested in how both would operate. But symbiotic relationships can be mutualistic, commensalistic, amensalistic or parasitic relationships; symbiosis is any one of those, not just mutualistic. I suggest you view this as it provides the textbook definition of symbiosis. Nonetheless, I enjoyed this answer, and as with the other non-accepted answer it has expanded my understanding of the topic. Upvoted. $\endgroup$ – blaizor Jan 25 '15 at 10:30
  • $\begingroup$ Good point. Then I meant to only talk about mutual symbiotic relationships. I'm kinda ashamed I didn't know the different types ;) $\endgroup$ – Sheraff Jan 25 '15 at 10:43
  • $\begingroup$ I would classify the example of ants protecting the tree is mutualistic symbiotic, not parasitic. $\endgroup$ – Paŭlo Ebermann Jan 25 '15 at 11:25
  • $\begingroup$ @PaŭloEbermann yes technically it is but I meant to use it to illustrate the "drug your host" strategy. $\endgroup$ – Sheraff Jan 25 '15 at 19:24
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I would say that nobility and peasants had a parasitic relationship in many societies in the history of mankind. This relationship was certainly acknowledged by both sides. It also wasn't a question of military might. After all soldiers beating down a revolution were peasants themselves and a united people could have gotten rid of the nobility at any point.

The way to perpetuate this relationship, even if it is clearly harmful for one party, is religion. "We all have our place in this world … and if god put parasites into our midst, who are we to remove them?" There could be some alleged spiritual benefits from suffering the parasites. Or possibly the parasites could function as a priest caste.

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  • $\begingroup$ Feudalism was a little more tit-for-tat; the nobles were educated and trained in military maneuvers at a strategic level; their knights, on a tactical level. Removing the nobles and knights from the feudal system would be like firing everyone in the U.S. Armed Forces above the rank of corporal, and leaving the rest to fend for themselves against the next invading force. In a region as populous and contested as Medieval Europe (or feudal Japan), not having someone to rally and lead the militia meant you were as good as dead, and there were few figures in history outside the nobility who could. $\endgroup$ – KeithS Oct 15 '15 at 23:14
  • $\begingroup$ Military necessity is a weak argument for feudalism. Choosing leaders and "special forces" by merit instead of by heritage would only improve matters. But anyway, this is about worldbuilding not about history or socio-economics. $\endgroup$ – BlindKungFuMaster Oct 16 '15 at 7:36
  • $\begingroup$ But it means it's a matter of a group over-charging for its contribution (specifically: demanding that its children be inducted into the group instead of the most able individuals regardless of birth), not literally parasitism. In that sense the two groups may not have fully appreciated the relationship, since plenty of people probably thought that birth was, if not a guarantee of suitability for the role born into, then certainly the best known proxy for it. $\endgroup$ – Steve Jessop Sep 12 '16 at 1:35
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In the Star Trek universe, the relationship between the Founders and their genetically-engineered creations the Jem Hadar has many of the qualities you describe. The Founders created the Jem Hadar race (I can't remember if they engineered an existing race or created them "from scratch") as soldiers, bred to be loyal, however like most intelligent underlings they were smart enough to wonder why they had to take orders. In answer, the Founders tweaked the engineering so the Jem Hadar are addicted to a substance, ketracel white, that can only be produced by the Founders and is controlled by the Vorta attaches (another genetically-engineered race, much physically weaker and thus much more naturally subservient to the Founders). Ketracel white withdrawal is a combination of the worst aspects of heroin withdrawal, alcohol withdrawal, dehydration and starvation, and is ultimately fatal.

Thus, the Founders more or less rely on the Jem Hadar to do their fighting for them, while the Jem Hadar have little choice but to fight and die for ketracel white. Very unbalanced on the part of the Founders; they provide very little and get a lot, so one could say the Jem Hadar are subject to a parasitic relationship with the Founders.

There was an episode of DS9 involving a Jem Hadar who inadvertently broke his addiction to the white by being stranded on a planet away from his chain of command, and convinced other warriors to come back to that planet with him to try to free themselves as well. The attempt ultimately failed, in part because Miles O'Brien saw the inherent danger of a race as powerful as the Jem Hadar being a "free agent" in the galaxy. Bashir, also part of the same episode, was trying to help them, believing that freeing the Jem Hadar would deprive the Founders of their primary and really only weapon.

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It sounds like a predator-prey relationship. Smart wildebeests being eaten by smart lions. The wildebeests have no choice but to migrate to where the good grass is. In stone age times they got picked off by lions along the way.

Now everyone is civilised and it is done as a 'tithe'. The wildebeest can choose to swim the Masai Mara river and get eaten by crocodiles and shot at by local hunters. Or they can hand over a tenth of their children and cross safely on this lovely bridge the lions have built.

Alternatively you are into brainslug or Stargate Goa'uld territory - the parasite species needs host bodies for part of its life cycle. Perhaps to take over the body. Perhaps to incubate their eggs in a live body, like a parasitic wasp does with a caterpillar. Though to be honest in both those situations it is a whole lot less trouble to possess a monkey or incubate your eggs in a cow. Monkeys and cows don't tend to have uprisings and rebellions.

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