anarchy, state and utopia

I find it extremely surprising that there's such a long section on animal rights in Robert Nozick's famous book "anarchy, state and utopia" but of course I'm pleased. I'll try blog on it in more detail later but for now I want to talk about the moral relevance of species membership. In discussions about animal rights I often find that people believe humans have value because of their special talents (things like language, moral agency etc), now I don't buy that argument but even if I did, what about all the humans who don't have any of those characteristics? People normally either point to their religious beliefs or argue that the very fact that these people are human is grounds for different treatment or consideration. If species membership is the crucial ingredient, would super intelligent aliens (with proportionally greater moral capacity) be justified in confining and killing us for their pleasure (or even for things like medical testing)? I can't really see why not. But what would we think of them morally as we were being led to the slaughter? If they knew how we can suffer surely it wouldn't matter how delicious we were.