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The human body is unable to do anything if it loses its head - as the brain is separated from the body, the body immediately dies.

But why does the same condition not exist for animals? Animals are similar organisms/living things, like humans. However, when the head is cut off from a chicken, snake, etc... the body continues to move. Why and how does this occur?

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    $\begingroup$ There is nothing different between animals and humans. Have you tried to google Why do headless chickens run?? $\endgroup$ – Remi.b Nov 11 '16 at 21:06
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    $\begingroup$ @Remi.b - Have you googled "Why do headless humans run?" Of course there's a difference. $\endgroup$ – anongoodnurse Nov 11 '16 at 22:23
  • $\begingroup$ @anongoodnurse My comment was misleading. Sorry about that. I reacted to the use of the term "live" as if in animals some fake essence of life of the organism was not dependent on the presence of the head while it would be in humans. $\endgroup$ – Remi.b Nov 11 '16 at 22:48
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In general, and this is somehow speculative, the animals that you mention have more autonomy in their extremities and less control from a head, muscles have evolved autonomous responses perhaps as an evolutionary advantage, ( i.e. having a brain be in charge of reactions can add precious milliseconds ), there also seems to be a relationship in between brain size ( specific motor control areas) and this autonomy, which introduces a metabolic cost (i.e. our big heads need a lot of energy) Further, there are other biological considerations for providing extremities with their own resources and thus to a degree be more independent, perhaps another evolutionary strategy. At the other end of this control-autonomy relationship, the head takes control of the extremities most of the time and their interdependance is greater, so if you cut it no movement will happen, although we still possess a few hard coded reflexes that work for a while provided they have enough energy left or a blood source.

To give you an example: a cat ( somewhere in the middle ) can still walk when you cut the connections from the brain /head to the legs due to reflex actions ( the cat has to be on a treadmill though).

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Some limb movements in vertebrates can occur at the level of the spinal cord (e.g. swimming behaviours in fish; coordination of alternate movement of legs in walking animals). This might support some uncoordinated movement for brief periods following complete severing of the head.

However, most (all?) vertebrates require nuclei in the brainstem to control breathing. Without an intact brainstem the body will not survive.

Decerebrate cat walking on a treadmill (but with an intact brainstem): https://www.youtube.com/watch?v=wPiLLplofYw

Chicken survives without a head (but with a brainstem): http://modernfarmer.com/2014/08/heres-chicken-can-live-without-head/

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Brain is needed for lung breathing motion (at least for mammals, maybe for all vertebrates), which limit the life span without brain.

Attention: In the case of the chicken, the low stages of the brain are at the level of the "neck", so sometime cutting the head is not fully cutting the brain and some base functioning can keep working.

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