Animal rights is a catchphrase akin to human rights. It involves, however, a few pitfalls. First, animals exist only as a concept. Otherwise, they are cuddly cats, curly dogs, cute monkeys. A rat and a puppy are both animals but our emotional reaction to them is so different that we cannot really lump them together. Moreover: what rights are we talking about? The right to life? The right to be free of pain? The right to food? Except the right to free speech all the other rights could be relevant to animals.
But when we say animals, what we really mean is non-human organism. This is such a wide definition that it easily pertains to potential aliens. Will we witness an Alien Rights movement soon? so, we are forced to narrow our field to non-human organisms which remind us of humans and, thus, provoke empathy in us. Yet, this is a dangerous and not very practical test: too many people love snakes, for instance and deeply empathize with them. Will we agree to the assertion (which will, probably, be avidly supported by these people) that snakes have rights or should we confine our grace to organisms with nervous systems (=which, presumably, can feel pain). Even better is the criterion : whatever we cannot communicate with and is alive is a rights-holder.
Historically, philosophers like Kant (and Descartes, and Malebranche and even Aquinas) did not favour the idea of animal rights. They said that animals are the organic equivalents of machines, moved by coarse instincts, unable to experience pain (though their behaviour sometimes might deceive us into erroneously believing that they do). Thus, any moral obligation that we have towards animals is a derivative of a primary obligation towards our fellow humans (the morally significant ones and only ones). These are the indirect moral obligations theories. For instance: it is wrong to torture animals because it desensitizes us to human suffering and makes us more prone to using violence towards humans. Malebranche augmented this rational line of thinking by proving that animals cannot suffer pain because they do not descend from Adam and all the pain and suffering in the world are the result of his sins.
But how can we say whether another Being is suffering pain or not? The answer is based on empathy. If the other Being is like us than surely he has the same experiences and, therefore, deserves our pity. The Jewish Talmud says: "Do not do unto thy friend that which is hated by you". An analysis of this sentence renders it less altruistic than it first sounds. The reader is encouraged to refrain from doing only things that he himself finds hateful (SS men, for instance, did not find killing Jews hateful). In this sense, it is morally relativistic. The individual is the source of moral authority and is allowed to spin his own moral system, independent of others. The emphasis is on action: not to DO. Refraining from doing, inaction, is not censored or advocated against. Finally, the sentence establishes an exclusive moral club (very similar to later day social contractarianism) of the reader and his friend(s). It is to his friends that the reader is encouraged not to do evil. He is exempt from applying the same standard, however lax, to others. Even a broader interpretation of the word "friend" would read: "someone like you" and will substantially exclude strangers.
Empathy as a differentiating principle is wrong because it is structural: if X looks like me, resembles me, behaves like me than he must be like me in other, more profound and deep set ways. But this is a faulty method used to prove identity. Any novice in mathematics knows that similarity is never identity. Structurally and behaviourally monkeys, dogs and dolphins are very much like us. It is a question of quantity, not...