How do zombies smell?

Question

In The Walking Dead, both the show and the graphic novels, it is established that the walkers determine prey from other walkers by sense of smell.

However, we see some pretty far decayed walkers, some of which are missing large parts of the face (I am pretty certain that I've seen some missing noses). Others seem to be missing most, if not all, of their respiratory system:

(note the exposed rib cage above)

It seems pretty established that walkers only need a head in order to survive. In the graphic novel, a severed head is seen to continue to bite and otherwise function.

How do walkers who don't breath, or who have seriously impaired breathing, distinguish prey from other walkers?

Answer 1

Along with a healthy suspension of disbelief, one theory goes that the zombies use whatever sensory organs they happen to have left to find survivors. Some number of zombies may not be able to use a particular sense, but others do - and if there's one thing zombies are good at, it's following each other to prey.

This is somewhat plausible given the numerous scenes (in both the comic and show) in which zombies are entirely inattentive, only to lash out after someone gets particularly close (entering range of whatever decayed sensory organs that zombie does have).

It's definitely not only the sense of smell - the reason not to fire a gun is that it will attract groups of zombies, so they're obviously using their hearing as well. Smell is key, however - there is a scene in the comic where Glenn and Rick go into Atlanta after covering themselves in zombie guts, and the zombies don't pay them any attention until it starts raining (despite them talking to each other).

Answer 2

How do walkers who don't breath... well, walk? They would need oxygen to use their muscles, right?

Any science fiction work requires some suspension of disbelief, especially if it involves zombies.

_{(My other theory is that a wizard did it.)}