Roger Scruton is a conservative English philosopher. He opposes animal rights because the "concepts of right, duty, justice, ..., responsibility, and so on have a sense for us largely because we deploy them in our negotiations and can invoke by their means the ground rules of ... social life, but which we can only use when dealing with others who also use them." Thus, he believes rights only apply to organisms that discuss rights, which is effectively equivalent to the argument that morality only applies to organisms that discuss morality. Scruton does not address the marginal-humans argument in this essay, but does excoriate Peter Singer, a notorious proponent of that argument; he also mocks Richard Dawkins and the animal-rights movement for "anthropomorphic and magical ways of thinking that science is supposed to dispel." This latter point is so obvious that, when I have mentioned it to acquaintances interested in the philosophy of "animal rights", several have gasped with astonishment that this concept had never occurred to them.
My current position is that an organism has rights when that organism has developed the minimum biological equipment necessary to engage in propositional logic (i.e. reason), regardless if the organism is currently utilizing propositional logic. The second half of the definition solves the marginal-humans problem for immature and impaired humans. At this time, homo sapiens is the only species with rights.