As regards animals, the position is clear. If an animal has the relevant moral capacities, actually or potentially, then it can be a possessor of rights. The evidence available to date about the rational capacities of animals is far from complete, but to date it appears to be decidedly unfavourable to the view that any animals possess the relevant moral capacities. Thus, whilst research on chimpanzees, monkeys, and many other animals, reveals a significant degree of rationality which provides an important ground for justified moral demands that they be better treated than they now are, the degree and kind of rationality fall far short of that necessary for moral judgment and moral self-determination. Although there is limited evidence in respect of certain animals of a capacity for seeming 'self-sacrificing', 'disinterested', 'benevolent' actions in limited, somewhat arbitrary areas, there is no real evidence of a capacity to make moral judgments, morally to discriminate when self-sacrifice, gratitude, loyalty, benevolence is morally appropriate, and more relevantly, to assess their moral rights and to exercise them within their moral limits. However, further research on animals such as whales and dolphins, although seemingly not in respect to monkeys, apes, chimpanzees, may yet reveal that man is not the only animal capable of being a bearer of rights. It may for this reason be morally appropriate for us meanwhile to act towards the former animals as if they are possessors of rights.
(H. J. McCloskey, "Moral Rights and Animals," Inquiry 22 [summer 1979]: 23-54, at 42-3 [italics in original])