I think you're simplifying the scope of this.Perhaps their slowness is less because of them being dead, and more a result of them being extremely careful about where they step? If I recall correctly, most of them seem to stumble around very slowly with their arms outstretched, like people walking through a dark room or people who have been rendered blind for the very first time.They are never shown bumping into walls,falling down stairs, or crashing into each other. Also the zombies seems way too coordinated for blind creatures who are only going by sound.Maybe it was so close to Hisashi's body that it could actually hear his pulse, and just bit in that direction?.If we also take into account the potential of them being able to feel differences in heat, rather than specifically touch, it could be that when they grab ahold of someone, or someone grabs ahold of them, they feel the heat coming off of them enough that it rings the internal dinner bell. And since the zombies basically move in the direction of whatever's there and bite, and often they seem to go for the head and neck if they can, it may be they're aiming for the sound of breathing and then bite whatever they grab onto. Typically when a fight engages with a zombie, unless someone is a particularly stealthy or calm fighter, there's usually going to be some amount of heavy breathing, yelling, or other sounds of exertion.Also, how do 'they' always know exactly where to bite? If 'they' truly only respond to sound and not touch, how did the one that bit Rei's boyfriend, Hisashi, know where his arm was?.We don't know that if one of them tripped over a metal bar and landed on some bubble wrap that the others WOULDN'T go crazy. They don't respond to footsteps because they're used to each other's footsteps, but living humans make all sorts of noises. Notice how they're never set into a frenzy by the human characters WALKING.

While they never had time to experiment with it, it could be that they can detect heat, which is why they only go after humans and not their dead comrades.

At least in the anime, Hirano asks Saya whether they can detect heat or not.

They can smell hormones? When did zombies ever make sense?.

So why doesn't the footsteps of their fellow not-zombies cause them all to start feeding on one another? In fact, their sense of hearing is so sensitive and their perception is so primitive that the sound of a tossed shoe or a wet towel hitting a wall can send them all into a frenzy.

The zombies are blind and can only detect ordinary people by sense of sound.

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Highschool of the dead tv tropes