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A while ago I was watching The Walking Dead (Season 1) with my sister, and we came across that one scene where Rick and his group theorize that zombies detect humans via smell, but here's the problem: ZOMBIES ARE DEAD (mostly) AND CANNOT BREATHE!! DUN DUN DUN!!!

Alternative Ways Zed detect prey:

Hearing: It's necessary but not enough, as the zombie cannot tell if it's coming from another zombie, a human, an explosion or whatever.

Touch/taste: No. Just No.

Sight: Maybe but I doubt dried up eyeballs are of any good or if the zombies are smart enough to tell if it's a human

So the question is to find a way zombies could accurately detect the presense of humans nearby so they could walk over and eat them, without relying on the senses above (Unless you could prove them reliable for zombies to use)

EDIT: THIS QUESTION IS NOT ABOUT MAGIC ZOMBIES! AS THE TAGS STATE THIS IS SCIENCE-BASED! I know how ridiculous actual undead zombies might sound like but we are getting closer!

EDIT NO.2: The zombies are partially alive, as their nervous system still exists. I still consider them undead because most of their bodily functions are down and they are recently dead people revived by the zombie pathogen. People have been reported to be saved 3 hours after they are supposed to die so I think it's possible.

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    $\begingroup$ I am not sure the 'zombies do not breathe' part is accurate, although I have never met a zombie to know for sure. Somehow, however, they need to metabolize something for energy in the muscles, don't they? $\endgroup$ Jan 3, 2023 at 4:24
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    $\begingroup$ That just means zombies don't have to breathe to supply oxygen, it doesn't mean they can't breathe for other reasons... like smelling things, or moaning "braaaaains". $\endgroup$
    – Cadence
    Jan 3, 2023 at 5:07
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    $\begingroup$ Actually, I believe the OP may be right. In damaged brains, smell is one of the first senses to go. Take a look at this post for some interesting food for thought. $\endgroup$ Jan 3, 2023 at 6:42
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    $\begingroup$ Zombies are necessarily magic... in the absense of magic, they just don't work. Therefore, the way they sense humans is magic. $\endgroup$ Jan 3, 2023 at 9:24
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    $\begingroup$ @Hi0401 nah, there's always magic, or pseudoscientific handwaving (which may as well be magic) or "sufficiently advanced technology" and we all know what Clarke said about that. Even in your previous answers you've ended up with "nanotechnology" which is just three different kinds of magic in a trenchcoat. Don't sweat it, that's just the nature of zombies. If you want zombies, you gotta put up with the magic, too. $\endgroup$ Jan 3, 2023 at 11:40

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Auditory Hyperesthesia

Induced by the unbearable Neuropathic pain associated with zombie physiology, zombies become hypersensitive to sound. They are now especially good at hearing and processing different sounds. Especially sounds that they are very familiar with, specifically the sound of a human heart beating.

In the question it says that hearing isn't enough, well it is now that it has been heightened and inadvertently trained (by the zombie's previous human life, hearing their own heartbeat). They can not only tell the difference between explosions, humans, and other zombies, but also the difference between a dog's heartbeat and a human's heartbeat. They can also determine the direction and approximate distance to the heartbeats that they hear, as well as any other sounds that the unfortunate humans make in addition to their beating hearts.

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  • $\begingroup$ Interesting. Sounds like something from ROTLD. I'll keep this in mind, may use it later $\endgroup$
    – Hi0401
    Jan 4, 2023 at 1:11
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Zombies smell using their skin.

Association between Olfactory Receptors and Skin Physiology

Olfactory receptors are chemosensory receptors that detect odorants and function in the initial perception of a smell. Intriguingly, olfactory receptors are also expressed in cells other than olfaction sensory cells, an expression pattern termed ectopic expression... This review introduces current research on the ectopic expression and function of olfactory receptors in skin and provides insight into directions for future research.

Humans have smell receptors in our skin. Sandalwood olfactory receptor is one the linked article discusses. The zombification process augments this. Olfactory receptors sprout up all over the zombie body. Exposure to air stimulates expression of these receptors. The zombie thus can smell just by air blowing across the body. As clothes fall off and wounds open up the surface area available for ectopic olfactory receptors increases. The sense of smell improves.

Zombies are attracted to human exhalations and other live human specific smells. Possibly they are attracted to zombie smells too but not as prey - Z likes to cluster because there is strength in numbers. This effect offers the possibility of smell bombs to attract away zombies. Also the possibility of treating zombies with attractive smells such that they will be attacked by other zombies and maybe attack themselves.

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  • $\begingroup$ Thanks for the answer. Just not sure if this will work if zombies had clothing on, or what they will resort to when their skin gets eaten by the zombification bacteria for nutrients. $\endgroup$
    – Hi0401
    Jan 6, 2023 at 1:40
  • $\begingroup$ I'm also not sure if this is enough to detect human activity as stated up there in the question, but maybe that's just because I am dumb :( $\endgroup$
    – Hi0401
    Jan 6, 2023 at 1:41
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They can't because, under the tag, they can't exist

I suspect you didn't read that tag's wiki before using it. It requires all answers to be entirely science-based. Zombies are dead. Olefactory doesn't work. Auditory doesn't work. Visual doesn't work. The Brain isn't working. The idea of a magic virus that can keep the brain alive without oxygen and can allow it to process complex signals simply doesn't meet the restrictions of the tag.

So, let's shift to the tag.1

Now, the problem with this is that as soon as you suggest that the zombie's ears and eyes are working, what's to say the nose isn't either? Muscles decay without oxygen so the lungs are still needed. Even if you suggest that whatever is sustaining the zombie is taking care of that, the reflex of breathing would still be built into the brain. (you have some control over your breathing, but for the most part, you don't breathe by choice.) So the sense of smell works just fine.

But, we want a solution that doesn't involve smell, well... because.

  • The process of zombification shifts the visual spectrum of the eye toward the infra-red. They can see heat signatures.

  • Zombification results in a form of autism, causing frighteningly high sensitivity to both sound and light.

  • The skin nerves are seriously heightened, causing pain even in a light breeze, but as a result, they can feel the motions of things around them. But it also means they can feel "dirty" in the presence of living humans in the same way that we often feel "dirty" in the presence of dead things.1

  • The virus (since we're reflecting science to some degree here) that causes zombification crates a nodule in the brain that acts in a way similar to the sensory navigation in birds: it detects the minute magnetic field caused by a large, living brain.


1I'm going to post a Meta question about modifying the three science tags to include the requirement that the tag can't be used unless the question also meets the conditions of the tag. It's not reasonable that a question should be allowed to use the or tags just because the OP think's it's fun to try and get a "better answer."

2Note that the human "feeling" or intuition I'm talking about is actually a complex reaction to a combination of sight, smell, taste, and touch. It's similar to the feeling of being watched. But we'll ignore all that for the purpose of this answer.

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  • $\begingroup$ The brain is alive. (But is mutated) and maybe the neurons throughout the body are too. But that's it. I haven't specified that yet, will edit $\endgroup$
    – Hi0401
    Jan 4, 2023 at 0:56
  • $\begingroup$ @Hi0401 I am sorry, but dead is dead. You can not posit a live brain in a body that can not support it. It needs a circulatory system to constantly replace the millions of cells that are dying hourly. Even if all that is functioning is the lower brainstem. Without some circulatory mechanism to continually replace these cells, your science based premise is as dead as the brain. $\endgroup$ Jan 4, 2023 at 2:45
  • $\begingroup$ Brain cells can't be replaced I think and I will address these problems in another question. I remember how cockroaches can live after being dismembered because of how blood is distributed around it's body, so yeah. $\endgroup$
    – Hi0401
    Jan 4, 2023 at 2:51
  • $\begingroup$ I don't really understand why everyone is talking about how a zombie won't be able to move with dead muscles. People walk when the brain fires signals to the muscles that result in the muscles contracting or something. This can be done to dead frogs so they could move after death. I've seen news/video clips about SKINNED dead toads jumping around because they are still being stimulated by the environment, why can't dead humans move in the same fashion? As for the muscle healing part I believe that muscles won't be damaged unless you use a certain amount of force... right? $\endgroup$
    – Hi0401
    Jan 4, 2023 at 2:55
  • $\begingroup$ @Hi0401 My primary point is that it's a bit unreasonable to ask for a science-based answer when you don't have a science-based question. Picking and choosing what science to use and what to ignore is what science-fiction is all about. Also, please note that there's a difference between a recently brain dead creature (chicken running around with its head cut off) and a dead creature (dead muscles). Once a cell dies, that cell can no longer contribute to contraction. Once enough cells are dead, the muscle can't contract even if a taser is applied. $\endgroup$
    – JBH
    Jan 5, 2023 at 2:06
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Motion and heat detection If we are talking about a zombie that is created via infection the only reason why they would "decay" like magic zombies is that whatever is infecting them is also consuming their bodies. With that in mind the sickness can pick and choose organs that gets consumed, so you could have a zombie who's brain has been partially consumed but still have advanced hearing or even sight. However the zombie would not have the cognitive ability to make sense of its enviorment again, due to its damaged brain so he would really on special triggers- if it moves like a human it can enter a state of hysteria- that would also trigger hysterical strenght. On top of that zombies having colder bodies is possible that they can feel heat from the distance- maybe even develop thermovision depending on the virus, so this way they can locate prey.

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OK! If you are looking for something with at least a pseudo-scientific explanation and level of believability, (pseudo, because... Zombies), then the Z's are most likely 'Infected' rather than 'Undead'. Infected could use any of the normal human senses, potentially further modified by the pathogen which caused zombification. If you really want to go with Undead/Reanimated, and insist that the eyes and nose dry up and that oxygen is not required for sense and mobility, then you can still potentially use Auditory cues - the ears will be less affected by dried up mucous membranes (since unlike eyes, mouth and nose they don't have any), and the sensory parts of the brain could conceivably continue to function minimally on stored fats, as in ketosis, rather than glucose and glycolysis. This would give the 'Undead/Reanimated' Z's a rather limited (un)lifespan of course, since they will run out of sensory neuron nourishment rather quickly. Maybe that's why they crave brains? If you want to go full-on Peter Hamilton/Charles Stross SciFi, then the neural systems of the Reanimated could be made functional by an extradimensional force, but then you aren't really dealing with Zombies, rather they become the Possessed.

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