We humans tend to under-estimate the intelligence of other species, or at least the level of their awareness. They may lack a common language and the ability for complex thought to put exactly what they are feeling into words, but they are sentient. Many of these creatures are capable of feeling the same emotions as we do, since we inherited an early form of our limbic and nervous systems from a common ancestor. These creatures have been shaped by time and nature, evolving along side of us, and possessing a consciousness in many ways similar to our own. Have you seen other animals, whether wild or a pet, express some of the same emotions as we do? They can express joy, sadness, and an almost infinite number of complex emotions in between (although in the wild the emotion that we most commonly invoke in them is fear).
What sets us apart from other more evolved mammals (primates, cetaceans, canidae) is our ability for complex thought and speech (made possible by our more advanced cerebrum). This is often used as the justification for treating them as lower forms of life. But is it right to discount the lives of other species simply because they lack our ability for rational thought? Is pain inflicted on these animals felt just as strongly as if it were another human being? It is possible that some of these species may possess a kind of awareness that we ourselves have lost?