It seems reasonable to suppose that dogs and cats have conscious experience, even in the absence of an ability to report. I am sympathetic, though, with Lloyd's remarks that this sort of thing seems to be more symptomatic of reflective consciousness than of primary consciousness. Of course the issue is complex, but strong intuitions suggest that a system could be experiencing while only thinking about the world, not about its own mental states. Sometimes it is suggested that conscious experience is a consequence of sufficient complexity , but this answer simply slides over the problem. Complexity is often relevant to the existence of some high-level phenomenon, but what is relevant is never complexity tout court , but the role that this complexity plays in a system.
Life requires complexity, because complexity is required for adaptation and reproduction.
If complexity is required for consciousness, it will be in virtue of some further functional property that this complexity enables, and we are seeing that this functional property is hard to pin down. Indeed, on reflection it is hard to see why the intuitions that lead us to ascribe conscious experience to dogs and cats should prevent us from ascribing it to mice, or to flies, or to simpler systems. When we look at the kind of processes that give rise to experience in humans, such as the processes underlying color vision, what seems most essential is that the processes make certain discriminations, and make the relevant information available to the overall system in the control of behavior. This is a very basic sort of information processing, and is something that even a very simple system might share. It is this sort of reasoning that makes it seems just possible, after all, that there might be something it is like to be a net, or even a thermostat.