Have you ever Stepped on a nail and not felt it--then: Pain Strikes you. You've been standing on the nail for a minute, but because you had your mind focused on the hottie walking by, you didn't notice it. This is revealing. It reveals that pain (the phenomenological pain) is a process of higher order functions. The "I" becomes aware of the of the pain and then it becomes "I-pain". In psychology, there is a distinction between aversive reactions and physiological response to a stimuli and the phenomenological pain response to a stimuli. Aversive reactions can take place without pain, but are many times accompanied by pain---emotional or physical, which are processed in the same area of the brain (see last months Scientific American). Now, humans and higher order animals can feel pain, but lower order animals may not feel phenomenological pain because they don't have the "I" concept or the ability the higher order brain functions to process suffering as anything more than a stimuli and response. When we talk about ethics with animals, we should consider degrees of suffering.
You might ask "How do you know lower animals don't suffer?"; well, I know what the necessary conditions for phenomenological pain in the human brain are, and I see them in some animals and not others, so I can say that I have a rational reason to believe that a chimp feels pain as I do, and a chicken does not because the chimp has the necessary conditions for phenomenological pain, and the chicken does not have the necessary conditions. Pain requires higher level function. Chickens do not have higher level, more associative functions, so they don't have pain---or there is no reason to assume that they do. If we start assuming that they do, then we could also assume that when I slap a tree it feels pain or when I cut a piece of neuronal tissue in a petri dish it suffers.
Now, consider, If I torture a Vegan and then torture a chimp, I have probably done something seriously immoral in both situations---however, something is is much more wrong in case of the Vegan. The chimp is alone, doesn't have higher order associations of "I" in culture, "I" in future, "I" in family or "I" in all of them and all of them in "I"; Because, it lacks higher order categories that come from language. It probably only feels, "'I' must escape, and endorphin/epinephrine rush, if the "I" doesn't break down all together in the face of the chemical rush because of the lack of other associations. The Vegan's "I" is more complicated, stronger, and capable of more "I" pain. He faces the "I" morally, the "I" spatially, the "I" linguistically, the "I" as self conscious field, the "I" culturally and the "I" within his kinship and friendship units. He is a stable "I" able to own his pain and multiple kinds of phenomenological pain, and there seems to be no escape from the pressure of "I" as found in those concepts. The Vegan is pressed as if by book ends into a unit by concepts.
I assert that when we consider the morals of animal suffering, there should be a consideration of the degrees of subjectivity. The same way that I don't assume slapping a tree or cutting a piece of tissue is torture, I shouldn't assume that butchering a chicken is the same as butchering a Vegan. There is simply no reason to assume that a chicken feels phenomenological pain. The aversive response is not enough. Science and ethics should unite on this front to discover which animals have the necessary conditions for phenomenological pain and protecting them.