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Realistically, how could a parasite become sentient? It doesn't have to be one that can control it's host like a yeerk from Animorphs, but that wouldn't be a disqualifying factor.

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    $\begingroup$ I assume you mean sapience instead of sentience. There are a lot of things working against you. Parasitism tends towards being very specialized, very asocial, and very static and stable living conditions all of which negate the need for intelligence. $\endgroup$
    – DKNguyen
    Apr 26, 2021 at 7:31
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    $\begingroup$ Sounds like a bad idea, since being sapient means your brain needs to be big enough and consumes a ton of energy, and if you ask me, a larger parasite that is depleting the host's resources faster isn't filling in the niche as well as the simple tick with a tiny brain which isn't stealing as many nutrients and resources and can stay hidden more easily. If you want an example of what it could be like just look at vampire bats, which are plenty smart already but certainly not sapient and will die in about 2 days if they don't have a proper meal until then, and they're 10 cm long. $\endgroup$ Apr 26, 2021 at 11:06
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    $\begingroup$ Semi-realistically... Would you categorize a Vampire as a predator, or a parasite? They are supposed to be very smart. There is no rule that a parasite must be a mostly sessile organism, bound to just one host. $\endgroup$
    – PcMan
    Apr 26, 2021 at 19:45
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    $\begingroup$ The modern, domesticated housecat is, impo, the most advanced parasite in the universe outside of ideological possession. $\endgroup$ Apr 26, 2021 at 20:50
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    $\begingroup$ @OscarBravo - We predator, not parasite. It is ludicrously rare for a parasite to drive a single species extinct, much less dozens. In general the population of parasites lags the population of their food. When food is abundant, parasites become abundant, but when food is scarce, the parasites die off. Pinnacle predators, or super-ultra-mega-predators, like humans, on the other hand .... $\endgroup$ Apr 27, 2021 at 14:55

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Realistically, unless the parasite can generate and perceive radio frequency signals, and therefore communicate 'telepathically' with other members of its species, there is little reason or opportunity for a parasite to become sapient.

Parasitism typically requires that the parasite lodge itself somewhere on or in its host's body, and simply attach and begin feeding, none of which requires much in the way of brains. Since the parasite doesn't kill its host, it remains up to the host to feed itself and its parasite.

The only likely way that a parasite could become sapient is for it to evolve to use its intelligence to assist its host, in which case it is no longer really a parasite but rather is a symbiote, or for it to use some long-ranged non-line-of-sight communication method to communicate with others of its own kind to their mutual benefit but not the host's benefit. Such communication abilities are unlikely to evolve except on a world where organisms incorporate metals into their bodies.

Since parasitism imposes selection pressures minimising the impact upon the host - at least until it is time for the parasite to reproduce and have its offspring parasitise new hosts - the selection pressures are typically those encouraging simplicity over complexity, and low energy over high, and sapience is typically both complex and energetically demanding.

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  • $\begingroup$ Would a vampire bat count as a parasite? $\endgroup$ Apr 26, 2021 at 15:02
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    $\begingroup$ @RossPresser Vampire bats are parasites; the only known parasitic mammals. They practice micropredation, a form of parasitism. en.wikipedia.org/wiki/Parasitism#Micropredator $\endgroup$
    – ceejayoz
    Apr 26, 2021 at 15:58
  • $\begingroup$ Very well. Today I learned. Deleting my mistaken comment now. $\endgroup$ Apr 26, 2021 at 21:31
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    $\begingroup$ @ceejayoz Curious, sincere question: are humans then practicing micropredation when we milk cows or shear sheep? $\endgroup$ Apr 26, 2021 at 22:35
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Intelligence came first. Parasitism came later.

Your species starts as a small meerkat-like carnivore that lives in groups - social cooperativity and intraspecies competition leads to intelligence.

A subset of this species associates itself with large herbivores. In addition to catching and eating creatures flushed out by the big grazers they clean the herbivores of ectoparasites. Some of these ectoparasites seriously decreased the genetic fitness of the big herbivores so that is a serious win for the herbivores.

Some of these creatures in addition to cleaning off parasites take a sip of blood or meat themselves. Over time this subset does less foraging and more sips of blood and meat from large animals.

This subset gradually evolves to complete parasitism of their original hosts and others. Body size decreases and they become stealthier. The intelligence evolved by their ancestors persists, but in a degraded, degenerate way.

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    $\begingroup$ Interesting evolution line. Maybe I can use that with some modification. $\endgroup$
    – Joe Smith
    Apr 26, 2021 at 21:17
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    $\begingroup$ The best parasite never surpasses the host. I really do agree with you about the degenerate form of intelligence, because being a parasite means you can stand on the back of the cow, but you could never fly like an eagle. Overtime, because the car was doing all the hard work, the parasite becomes a smaller and smaller thing. If it’s big enough that I can go head to head with a cow, the cow is probably a lot more involved at butting heads. $\endgroup$ Apr 28, 2021 at 2:11
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Spreading and feeding

There is much literature and other media like games where you can see why parasites would want to get smart, even if it isn't said explicitly. This also has a evolutionary perspective, which we can actually see in nature. Like all organisns they want to spread.

Parasites exist in many forms. From fat worms that can become several meters long to viruses and bacteria. Some use existing 'biological infrastructure', like feeding, breathing or excrement, to spread. But some use some extra assistance to either protect themselves or help spread. A bacteria that uses cats to multiply can infect mice and make them lose their fear for cats. Rabies can cause fear of water to such an extent that spasms are triggered when trying to swallow. Cordyceps will influence ants or other insects to move to certain locations, grip until they die and then the fungus will grow out and spread. Each is a mechanism to control. This control is to try to spread better or for defence.

From there it really isn't a big leap for a parasite to start controlling more and more of the actions. First it might be to spread better and better, but at a certain moment the host might be more protected as well. The longer the host survives, the better it can spread. The parasite would start to improve food intake and possibly blend better into the society it works in, so it won't be rejected by other potential hosts and can stick around. The intelligence might not come from the parasite, or at least not at first, but might gain this ability at a later stage.

After a certain amount of control is established, the parasite/host organism can nearly be seen as one entity. That means that, if applicable, intelligence is the right strategy for further evolution. Meaning they can take the same evolutionary path as humans, because it benefits them.

It is a long, complex road to such parasites and they likely only infect one or two species at first. It is unlikely. On the other hand, humans getting this much intelligence was also unlikely. We see it could happen with humans, so the same could be valid for parasites.

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    $\begingroup$ The difference between a change in the host's behavior and the parasite becoming a symbiotic co-processor is quite significant, and the selection pressures on parasites are counter-complexity in nature - a small, simple, low-energy parasite is less likely to kill its food supply than a large, complex, high-energy parasite. $\endgroup$
    – Monty Wild
    Apr 26, 2021 at 9:41
  • $\begingroup$ @MontyWild♦ the same selection pressures can be said for complex multicellular life in general, or intelligence specifically. In times of abundance it can still arise and proof worthwhile for any organism. $\endgroup$
    – Trioxidane
    Apr 26, 2021 at 9:48
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Animal Husbandry based Parasitism

Humans used our intelligence to domesticate various animals such as goats, sheep, and cows. To these animals who we use for milk, we are by every definition parasites. In some cases, humans like those of the Steppe peoples have also been known to drink the blood of their horses while riding them.

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    $\begingroup$ In the general case it is a symbiosis. $\endgroup$
    – fraxinus
    Apr 27, 2021 at 6:33
  • $\begingroup$ Sentient parasites would go a long way towards enabling symbiosis since they won't want to depend on the randomness of their hosts. $\endgroup$
    – alamar
    Mar 11, 2022 at 14:54
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Others have gone into detail about evolutionary factors which work against the evolution of higher intelligence in a parasite. There are still ways for parasitic life to "find a way."

Another scenario which may work in conjunction with Willk's idea. Perhaps parasitism is only one stage of the life. Or perhaps the parasite lives inside a creature large enough to provide enough calories to support the parasite's sophisticated nervous system.

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  • $\begingroup$ Hello! When answering questions it is often a good idea to provide resources or links to help further explain your response. $\endgroup$
    – A Writer
    Apr 26, 2021 at 19:31
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The book "Parasite" by Seanan McGuire (author of somewhat schlocky urban-fantasy) has genetically engineered tapeworms doing this. Everyone has one now and they keep you super-healthy with their ability to adapt.

But in rare cases, especially for a brain injury, they can spread to the brain, "adapt" to it and accidentally take it over. Of course it's like a new brain -- they have to learn how to walk and talk, but are fast learners.

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    $\begingroup$ Preceded by Futurama. And Blood Music too. $\endgroup$ Apr 26, 2021 at 22:36
  • $\begingroup$ @RossPresser Those Blood Music things seem like simply nanobots (yes, yes, at the time it was new), whereas tapeworms are actual parasites and they themselves get smart. BloodMusic make it sound as if the humans become more intelligent with nanobot help. $\endgroup$ Apr 26, 2021 at 23:26
  • $\begingroup$ Well, for most of the book the viewpoint is that of the humans, and the Blood creatures just seem like an alien invasion, albeit one that improves human individuals. But one of the threads of the story follows a human who is fully downloaded into cells, conscious, and meets others. They aren't human minds exactly but they are minds, not just machinery. $\endgroup$ Apr 27, 2021 at 1:48
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Perhaps a parasite that integrates with the brain/nervous system, like a Neural Parasite from star trek, could become sentient upon integrating with the host.

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