This is crossposted from my elljay.
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So I have a theory that Yeerks are not as resilient as they think to the cries of a host and/or the emotions and adrenal system of a higher-functioning species. Visser Three (Or Esplin 9466, The Prime) is the only andalite-controller, and even though a dedicated and well-read Animorphs fan knows he's a brilliant strategist and planner, he's also evidently quite stupid in times of action and genuinely insane all of the time.
I think that KAA planted enough hints in Visser that this is a valid interpretation. Edriss muses on the fact that she became so addicted to the emotions of humanity that it affected her perception and judgement. Enough so that she was hauled before the Council and charged with Treason.
In book 26 when the Anis are transported to the Iskoort planet, Ax is the most heavily affected of any of them by the howler's roar. It is explained that this is because of Andalite's higher cognative abilities. I wonder, if just being in a Human is enough to drive Yeerks loopy, what would having an Andalite host do? Amplify the Yeerk's own arrogance, perhaps? Make him stew in his own bitterness, rage, and anger because he cannot separate himself from his host?
These are guesses, but I think that sub-visser Taylor gives readers a much-needed bridge that completes this theory. The Yeerk who took Taylor quickly began to lose it, wrapped up in Taylor's teenage angstfest of pain and rage from going from rich and worshipped at school to scarred and mangled in a house fire. It was unable to separate itself from her personality, warped and distorted as it was with narcissism and black, tarrish anger.
In book 6, Jake is taken as a host. The Anis have to hold him for three days to starve out the yeerk. During this time the readers and Jake both learn that Yeerks imprint a part of the Host's personality onto themselves and carry it with them forever after, like a living sponge.
THIS IS VERY ****ING IMPORTANT INFORMATION. It tells us that the yeerks as a species have no choice but to absorb traits of their hosts as they sift through memories and thoughts. This also means that just as memories and emotions are retained as an imprint and used as part of the yeerk's own mind, so are the host's own logical inconsistencies and personality defects.
If the host is a paranoid schizophrenic with Grand Mal delusions, I wonder: how would the Yeerk react to auditory or visual stimuli with no grounding in reality, i.e, hallucinations? How would it deal with the constant dread of They're watching me, always watching when for the yeerk this is a real threat in the chain of command? I think the mental deterioration of both host and yeerk would be slow, but genuine. And
permanent.
I also wonder: What is the effect on the mental health of the Yeerk once the Host has been broken and no longer believes rescue is possible? That is definite mental and psychological trauma, and it's probably not present in taxxons (because they are allies and mostly driven by hunger) and gedds (because they're barely sentient) and only minorly there in Hork-bajir (who are not very bright crayons in a box of neon colors). If the host believes he/she is truly always doomed to be a captive you're either going to get a stockholm syndrome effect (one reason I have for voluntary controllers is that after seeing how hopeless it is, the host eventually turns voluntary just to get some control back once their yeerk is feeding) or they'll start developing signs of cabin fever that further exacerbates into true mental instability. Day after day, hour after hour, being able to not cry out to a loved one only a few feet away? To not be able to move your eyes? To hear the malicious laughter and threats and horrors your captor heaps upon you to keep you downtrodden? A normal, healthy person would start to lose their grip within weeks.
Thoughts?