@Fido,
I enjoy Peter Singer's utilitarian position on this subject. Let me preface what I'm about to say by mentioning that I've only read one of Singer's books, "Animal Liberation," and I read it probably over a year ago so what I say could be completely off base.
Singer argues that humans participate in speciesism when they, among other things, consume non-human animals. If i remember correctly he defends a vegan stance because plants cannot feel pain whereas animals can. He highlights how non-human animals have the same basic framework as humans do in regards to a nervous system. From this he infers that animals feel pain just as we do, and that pain is a bad thing. He thinks that it's our role to reduce suffering (utilitarianism), and the main way to achieve that is by not eating/testing on/hunting non-human animals.
I like his position but it raises a lot of other ethical issues. For instance, if you were to defend Singer's position you would have to say that a human "vegetable," for lack of a better term, is more expendable than a deer. If this human had no ability to feel pain, harming the deer would be the worse action.
I hope this helps explain at least one stance on the subject of speciesism.