Carl Sagan talks a little bit about Schizophrenia in his book Dragons of Eden. I've held for a long time now that what we experience as "consciousness" is just an evolutionary adaptation and what we call Schizophrenia is this same adaptation just gone a little haywire. I believe animals, mammals in particular, are fundamentally no different in this regard, it's just that the sophistication of their inner voice is likely to just be far inferior to ours due to our brain/body ratio and the development of the neocortex in humans. For me, personally, I feel that when I speak to myself in my head, the inner-chatter I think of as consciousness, I also feel as though I'm speaking words through my mouth, I'm just not using it. Maybe language also has something to do with it? Maybe we developed language and "consciousness" together?
Just a pet personal theory, could be likely very wrong and I'm sure since this is the Internet somebody will be happy to oblige.