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I know from the Yeerks' experiences with the Hawjabran that the Yeerks can only take as hosts species that have a central, unified nervous system.

But are there any other limitations?

  1. Does the organism have to have a certain amount of intelligence? I know that the Gedds had limited intelligence, but they could still be taken as hosts. Is there a lower limit? Are Gedds the "dumbest" creature they have taken? (I know this is subjective.) What about a human with Down Syndrome, or an even worse mental disability?

  2. What about physical brain size? Does the brain have to be a certain size for the Yeerk to "fit" in there? What about body size in general?

  3. Does age matter? I know that the Yeerks infested human children, but how young? This also tells us something about #1 and #2, as children have physically smaller bodies and brains and are generally less intelligent (in many ways, not all) than adults.

  4. Are there any other criteria for organisms that the Yeerks could successfully take as hosts? (I mean what they could do, not what they would necessarily want to do.)

  • There's some info here including a list of host species, but not very much. – Rand al'Thor Jun 21 '16 at 10:20
  • @Randal'Thor Thanks for the list and notifying me of the spelling errors. – Revetahw Jun 21 '16 at 10:23
  • One particularly noteworthy species - especially for this question - that's left off the list @Randal'Thor posted is "modified hammerhead sharks." Being a species that originally wasn't suitable but was modified to be, that's probably one of the best places to look for a picture of some of the requirements. – Mike Kellogg Jun 22 '16 at 14:14
  • @MikeKellogg This is certainly worthy of its own answer. – Revetahw Jun 22 '16 at 14:18
  • Maybe once I have time to refresh my memory on what exactly those changes were, if someone else doesn't take it first. =) – Mike Kellogg Jun 22 '16 at 15:36
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Adding to your previous answer, the Yeerks have a few other requirements for infestation beyond size:

The Yeerks couldn't infest the Arn due to a weak blood vein in their heads which would burst if a Yeerk entered. From this we can assume the obvious: the Yeerks can't successfully infest something that would die if infested. Moreover, this shows an additional requirement: the Yeerks require a path to the brain -- they didn't create new paths to make up for the Arn's unsuitable one, and due to their need for Kandrona rays they couldn't surgically implant in a host.

The Yeerks couldn't infest the Venber due to their extremely low body temperature ... and from this we can tell that Yeerks require hosts with hospitable internal environments.

The Yeerks couldn't infest the Hawjabran due to their distributed nervous systems. From this, we can tell that the Yeerks require hosts have a central nervous system.

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Regarding age, a five year old host is encountered in #29.

Regarding size, in #2 The Visitor Visser Three is talking about a cat (emphasis mine).

"What a ferocious little beast," Visser Three said approvingly. "See how he did not back away or run? I am many times his size and yet he struck at me. A pity that the species is too small to serve as a host."

(Animorphs #2: The Visitor, p. 57)

We know that horses are big enough to be taken as hosts, but this comment indicates that cats are not.

But is the Visser talking about brain size or body size?

An average cat's brain weights 25 grams and includes 760,000,000 neurons.

An average horse's brain weighs 655 grams and includes 1.200.000.000 neurons.

It is thus inconclusive. Visser may either be referring to the body or the brain, or both. But since the Yeerks stay only in the brain, it's possible that the brain size is the only criteria. Visser had seen the body of the cat, but could he have known how big its brain was? Moments before, the Visser had asked:

What is that?

to which Chapman had replied:

It's called a cat. (...) An Earth species used as a pet. The humans keep them close and find comfort in them.

So it was the first time Visser Three had seen a cat, and he couldn't really have known how large a cat's brain is. Though he might have been able to judge solely from the size of the head, if he didn't know about cats, he could hardly be an expert on Earthian biology nor Earthian brain sizes relative to the head. Maybe he yoogled it (Yoogle, Yeerk search engine.) After all, he was online (or whatever Yeerks call it) at the time, talking to Chapman. Seriously, though, this seems unlikely.

Since Horses were fit for infestation, it suggests that the intelligence requirements are relatively low. An (even moderately) mentally disabled human should be no problem at all. (Just guessing, though.)

I realize this is only a partial answer. Still looking for more comprehensive details from other users.

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    ROFL at "Yoogle, Yeerk search engine". – Rand al'Thor Jun 21 '16 at 15:06
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    Remember that the creatures the Yeerks evolved to infest (Gedds) were small, dumb, and slow. – Slacklord the Terrible Jun 21 '16 at 17:27
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    Well as far as cats go i'd assume Visser three simply eyeballed it and decided that the ear canal wouldn't be big enough – IG_42 Jun 21 '16 at 17:46
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    @IG_42 Yes, that's possible. I agree that Visser probably said it casually after a brief glance at the cat. But I remember reading that the Yeerks are extremely flexible, as they have to twist themselves around every part of the victim's brain. But maybe the cat's ears would indeed be too small. I guess if Visser really wanted to infest cats, he could surgically alter them a little bit, or go for lions. – Revetahw Jun 21 '16 at 17:50
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    @IG_42 Haha... Yeah.. quite a scene at the Yeerk pool when the Lion controllers need their kandrona... I can picture 4 Hork-Bajirs trying to tie up a lion and press its head down to the pool without getting bitten, lol. – Revetahw Jun 21 '16 at 17:54

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