I think that's the point - they are attacking each other when its "feasible" (if I can use that term) - recall somewhere in episode 10 I think when Angel and Riley are rescuing Datu from the arena - they see the wave running - he says they're not running "towards" us, they're running "from" something - that was the (my) first indication that some zombies feed on othersQuote:
But it they are starving and they are cannibals, then why don't they attack each other?