In addition to an explanation of utility, however, there is another integral part to the principle of utility; this is its scope. It must be made clear that it is the act's consequences as affecting everyone and not just the agent himself which are to be considered. For, at least as both are usually construed, the only major difference between ethical egoism and act-utilitarianism is that the egoist is concerned with maximizing utility in his own case, so that only consequences which affect him bear upon the rightness and wrongness of his acts. In an act-utilitarianism, on the other hand, everyone affected by the act is to be considered, and to be considered equally, at least on the usual assumption of each to count for one and none for more than one. But if everyone affected by the act is to be considered, are we to consider the animals affected by our acts? As we saw earlier, virtually all utilitarians, present-day as well as classical, have wanted the scope of the principle of utility extended to animals, or, in any event, to the 'higher' animals. In this way, we obtain a characterization of the principle something like 'Always maximize net satisfaction of interests of animals as well as humans'; and this expanded characterization both accurately reflects present-day views on the interpretation of utility and on the scope of the principle of utility and is, in its present essentials, a plausible candidate for the principle of utility.
(R. G. Frey, Interests and Rights: The Case Against Animals [Oxford: Clarendon Press, 1980], 132)