How does killing a host kill the Yeerk too?

[Dogman15]

Does the Yeerk die when the host's brain stops receiving oxygen? But the Yeerk is an independent life form that uses Kandrona for energy, and I don't think Yeerks need oxygen, because they swim in a liquid usually. Unless they have a way of getting oxygen out of the Yeerk Pool water like fish, amphibians, and other aquatic-related species get oxygen.

So maybe the Yeerk gets the oxygen it needs from the host's circulatory system via blood vessels to the brain. Then when the host dies, this gets cut off and the Yeerk suffocates.

Any other ideas?

[Nar Klawip]

I'm pretty sure that the Yeerk died when it's host did.

It just made sense to me that if your yeerk is tied in to your brain's synapses and neurons and those shut down then the yeerk shuts down with it.

It's kind of like when you have a microphone plugged into your computer, when you shut down the computer the microphone turns off.

I always assumed that was why the yeerks always tried to leave their hosts before they died; so they wouldn't die with them.

I never really thought the yeerks required oxygen, at least not in the same way as humans, since in their natural state they are aquatic slugs.

[wildweathel]

Physiologically, Yeerks are very much like sea jellies and like sea jellies the seem to be able to survive in deep or unagitated water with very little oxygen.

But when actively controlling a host, that involves a large amount of neural activity, and consequentially a lot more oxygen--conveniently supplied by the host. 

That suggests that unhosted Yeerks live in a semi-coma to conserve oxygen.  Not only is the pool lacking in stimulation, it simply doesn't have enough oxygen to support rapid thought.  (The more I think about the poor slugs, the sorrier I feel...)

Anyway, a dying host is not a good place to be.  In addition to the neurological shock ("severe" must be an understatement; we know they can feel pain when they're 'plugged in'), circulatory failure rapidly leads to hypoxia as the host's brain sucks up what little oxygen is left.  The Yeerk has to avoid losing consciousness, get out*, and get somewhere reasonably moist to survive on precious, precious little oxygen.

(*A rather delicate operation.  Look how tiny those holes are!
[spoiler][/spoiler])

So, "how does killing a host kill the Yeerk, too?"  In a horrible blind suffocating panic, that's how.

[Chad32]

I think that makes sense. The brain is trying to survive as long as possible, so it's pulling the oxygen it needs from every source possible, including a Yeerk that's attached to it. The dying brain will kill the Yeerk.

[AniDragon]

A lot of it could be shock, too. If you feel enough pain (which, dying tends to hurt), you might go into shock, which for all we know could be fatal to Yeerks. *shrugs*

[TobiasMasonPark]

     I just assumed that, because they're attached to the brain, they just shut off once the host dies.

[wildweathel]

Oh, and the Yeerk probably needs a higher concentration to stay active than the host needs to stay alive, so the dying brain tissue sucks the oxygen out of the Yeerk that it would need to escape, but not enough to kill it outright (they probably can survive some incredibly low levels).

And, as far as we know, Yeerks don't ever completely lose consciousness. They don't sleep, and (however slowly and groggily) can think even in the depths of the pool.  

What a horrible way to go.  Just like drowning after an extremely intimate experience with one's host's death.

It's a good thing Cassie never really thought about this....

[Myitt]

I actually never thought of it as an oxygen thing; first of all, Yeerks don't live in deep or undisturbed water, they live in shallow pools that have a circular current (described in a few books where the Animorphs are IN the pool) and are a roil of activity from swimming Yeerks.

Second of all, just because a Yeerk lives in water doesn't mean it isn't consuming oxygen.  There is oxygen in water, hence the O part in H2O.  It could simply be occurring through direct exchange across a mucous membrane, which the Yeerks seem to be pretty much surrounded by.  When Cassie morphs Illim in book 29, she still feels like she's breathing and she guesses it's through her skin.  Perhaps Yeerks are able to slow down their need for oxygen while in a host, or maybe the neural connection is also an oxygen exchange, but I think sapping oxygen from a host's brain would be a pretty damn big health risk for the host.  

Long story short, here's what I think.  Might not be right, but to me, it seems most plausible.  Yeerks are very small, but they're very smart.  Likely explanation?  They have a pretty large brain-to-body-size ratio.  Big brains in a little body.

Yeerks also connect their bodies, their neurons, directly to the host neurons.  

Explanation?  Well, it seems that the Yeerks are mostly brain, and not a centralized brain like ours--their whole bodies need to be able to flatten out and connect a huge net of neurons to neurons on the host brain's surface.  

So if your entire brain and body is connected to another brain and body, if that other, host brain stops functioning your little sluggy body wrapped putty-thin around it has very, very little time to disengage before death reaches you as well.

[Unknown User]

Yeah, but myitt, death is not a force that spreads, death is the absence of other forces...It's not like it is a disease that can spread to the yeerk. It's just the result of something turning off.

I like the oxygen theory, but I adamantly disagree with wildweathels idea that yeerks are in a psuedo coma when not in a host. All you have to do is read the chapters of THC when Esplin is in the pool learning about andalites to know that there is no coma involved.

[Myitt]

I disagree about the near-coma theory too, Yeerks are shown to talk in pools several times, and they swim actively.

As for your comment on death, my point is that death CAN spread to neurons that are connected to other, dying neurons, in the same way that the brain itself dies neuron by neuron.  The Yeerk's brain and body is just connected to that chain reaction, and in a way the ability the Yeerk has to control the host body also ties it into the host's pain, and the host's death unless the Yeerk is very quickly detached.  That direct of a connection between Yeerk and host, I think, makes it very difficult to escape a host before "death reaches the Yeerk as well" as pointed out in Visser.  From a biological perspective it makes perfect sense to me; when the host shuts off, so does the thing that has directly connected its whole mind, its life, to that dying connection.

[wildweathel]

Yeerks don't live in deep or undisturbed water, they live in shallow pools that have a circular current (described in a few books where the Animorphs are IN the pool) and are a roil of activity from swimming Yeerks.

Okay, time for a little chemistry.

Does mixing the water help?  A little.  It pulls oxygenated water away from the surface and replaces it with less oxygenated water.  The lower average concentration at the surface means less dissolution back into the air.  Maybe good for a 10% increase in rate; more for deeper pools.

But the big factor would be surface area.

An adult human has about 160 square meters of respiratory surface area.  At rest, about 20% of respiration goes to the brain; that's 32 square meters.  46 cubic centimeters of oxygen per minute.

That's a lot more real estate than your typical pool has.  An olympic-sized swimming pool (hey, I had to go there) has enough surface area to support about 40 human brains (wow this is getting weird).  So the question is how big's a Yeerk? 

[spoiler=sources]
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pulmonary.html
http://faculty.washington.edu/chudler/facts.html
[/spoiler]

Well, if we figure they're about the size of a big banana slug, that's somewhere in the neighborhood of 70 to 100 grams.  If we use that figure of 3.3 cc/min/100g and a 70g Yeerk, that's 2.3 cc/minute.  5% of a human brain.  Our olympic-sized swimming pool has surface area for 800.  Not exactly a "mass," more a "sprinkling."  Over three cubic meters of open water for each.

(3,200 liters and a mule...)

There is oxygen in water, hence the O part in H2O.

Wrong type of oxygen.  It has to be diatomic O₂, otherwise the chemistry doesn't work.

she guesses it's through her skin.

I agree.  That's how banana slugs breathe.

sapping oxygen from a host's brain would be a pretty damn big health risk for the host.

Not really.  There's a large blood supply constantly delivering oxygen to brain tissue.  An extra 5% load isn't going to break anything.

it seems that the Yeerks are mostly brain

All brain/muscle without tissue differentiation, I'd say.  Like jellies. 

I like the oxygen theory, but I adamantly disagree with wildweathels idea that yeerks are in a psuedo coma when not in a host. All you have to do is read the chapters of THC when Esplin is in the pool learning about andalites to know that there is no coma involved.

Maybe "coma" is the wrong word.  I'm thinking of how we use induced comas to reduce brain metabolism by 30% or so when treating brain injuries.  In humans, that involves a loss of consciousness, but imagine Yeerk tissue is adapted to only think as fast as available oxygen allows.  They can cut oxygen use by 99% or better compared to what a mass of human brain tissue would require, but only a tiny bit of that is increased efficiency.  The rest is made up for by t-h-i-n-k-i-n-g v-e-r-y s-l-o-w-l-y i-n t-h-e p-o-o-l.  This is not something you'd notice just by experiencing it; without something to compare against, how do you measure time?


Oh, and death is complicated and messy, it's not entirely like there's an "okay everybody, time to die" signal from cell to cell.  But it's not entirely unlike that either.  http://en.wikipedia.org/wiki/Clinical_death

[Myitt]

Okay, time for a little chemistry.

I've had four years of it in my life, I'd rather not have any more XD

I'm not big on the oxygen idea personally, so I'll skip commenting on the chemistry--but it sounds convincing.

There is oxygen in water, hence the O part in H2O.

Wrong type of oxygen.  It has to be diatomic O₂, otherwise the chemistry doesn't work.

How do you think fish breathe?  We don't need to lecture each other here.  Yeerks probably have a way to split water to form diatomic O2 just like fish do, or like slugs, through direct skin exchange.

sapping oxygen from a host's brain would be a pretty damn big health risk for the host.

Not really.  There's a large blood supply constantly delivering oxygen to brain tissue.  An extra 5% load isn't going to break anything.

Suit yourself!  I'd rather have my full brain quota of oxygen, especially at this altitude...

it seems that the Yeerks are mostly brain

All brain/muscle without tissue differentiation, I'd say.  Like jellies. 

That's pretty much how I picture them: A nerve net rather than a concentrated ganglion.

[Blazing Angel]

To completely take over the yeerk has to connect directly to the hosts brain nearly becoming one with it. So its like in old sci-fi movies where someone is connected to a computer and something goes wrong and their stuck. Maybey not exactly like it but you see my point.