Other animals, which, on account of their interests having been neglected by the insensibility of the ancient jurists, stand degraded into the class of things. … The day has been, I grieve it to say in many places it is not yet past, in which the greater part of the species, under the denomination of slaves, have been treated … upon the same footing as … animals are still. The day may come, when the rest of the animal creation may acquire those rights which never could have been withholden from them but by the hand of tyranny. The French have already discovered that the blackness of skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor. It may come one day to be recognized, that the number of legs, the villosity of the skin, or the termination of the os sacrum, are reasons equally insufficient for abandoning a sensitive being to the same fate. What else is it that should trace the insuperable line? Is it the faculty of reason, or perhaps, the faculty for discourse?...the question is not, Can they reason? nor, Can they talk? but, Can they suffer? Why should the law refuse its protection to any sensitive being?… The time will come when humanity will extend its mantle over everything which breathes…
Mill distinguished between higher and lower pleasures to avoid the charge that utilitarianism is “philosophy for swine”, but still wrote, from that Wiki page section you cite,
Granted that any practice causes more pain to animals than it gives pleasure to man; is that practice moral or immoral? And if, exactly in proportion as human beings raise their heads out of the slough of selfishness, they do not with one voice answer ‘immoral’, let the morality of the principle of utility be for ever condemned.
The section also doesn’t actually mention any theories for “Humans alone”.
I’d also say that utilitarianism is often grounded with a theory of utility, in such a way that anything capable of having utility in that way counts. So, there’s no legwork to do; it just follows immediately that animals count as long as they’re capable of having that kind of utility. By default, utilitarianism is “non-speciesist”, although the theory of utility and utilitarianism might apply differently roughly according to species, e.g. if only higher pleasures or rational preferences matter, and if nonhuman animals can’t have these, this isn’t “speciesist”.
Classical utilitarianism, as developed by Bentham, was anti-speciesist, although some precursors and some theories that followed may not have been. Bentham already made the argument to include nonhuman animals in the first major work on utilitarianism:
Mill distinguished between higher and lower pleasures to avoid the charge that utilitarianism is “philosophy for swine”, but still wrote, from that Wiki page section you cite,
The section also doesn’t actually mention any theories for “Humans alone”.
I’d also say that utilitarianism is often grounded with a theory of utility, in such a way that anything capable of having utility in that way counts. So, there’s no legwork to do; it just follows immediately that animals count as long as they’re capable of having that kind of utility. By default, utilitarianism is “non-speciesist”, although the theory of utility and utilitarianism might apply differently roughly according to species, e.g. if only higher pleasures or rational preferences matter, and if nonhuman animals can’t have these, this isn’t “speciesist”.