The answer to this question will depend on how good the evidence is for saying that various animals do or do not have the mental capacities which each thinker takes to be morally relevant.

Note that all answers to questions about animal minds (and, for that matter, human minds!) are based on arguments by analogy, which have this basic form:

• Things like X are known to have qualities a, b, c, . . . and m as well as n.
• Things like Y are known to have qualities a, b, c, . . . and m.
• So things like Y probably also have quality n.

When the question is, "What animals have mental states of type n like those humans are known to have?" the analogies appealed to -- a, b, c, through m -- will be various relevant behavioral and neurophysiological comparisons.

We saw that Singer's utilitarianism is complex, endorsing a preference-bassed conception of happiness for animals who are capable of conceiving of their own future, and a hedonistic conception for those who are not. Here we only consider the question of which animals count from the moral point of view from the perspective of hedonistic utilitarianism. (Regarding preference utilitarianism, see the discussion of desires below.)

On this view we ought to maximize aggregate happiness, where happiness is conceived in terms of feeling pleasure and avoiding pain. So on this view all sentient animals (all those capable of feeling pleasure and/or pain) count.

Singer himself says this includes:

• at least all vertebrates, but

• perhaps not crustaceans and probably not animals as simple as oysters (see Animal Liberation, p. 173-74).

The are complex conceptual and experimental issues here, but the authors of the most detailed treatments of the question, "Which animals are capable of feeling pain?" have all reached the conclusion that while all vertebrates probably can feel pain, most invertebrates probably cannot (the notable exception being the cephalopods). For more on this issue, click here.

Since Regan conceives of harm in terms of diminution in the capacity to form and satisfy desires and attributes rights to any individual who is capable of being harmed in that sense, the scope of moral rights for Regan is going to be all animals which are capable of forming desires (which are, note, the kind of mental state that preference utilitarians are concerned with).

Regan himself says this includes:

• at least all normal adult mammals of a year or more, but

• probably also many, and perhaps all, other vertebrates (see The Case for Animal Rights, pp. 78 and 367).

Regan limits his own consideration of the relevant evidence (chapter two) to mammals, but only to avoid the "line drawing" issue of where exactly the capacity for desire disappears (p. 78). Here again there are complex conceptual and experimental issues, but elsewhere I have offered a detailed defense of the view that while all normal mammals and birds probably have desires, fish and invertebrates may not. Click here for some of the relevant considerations.