Preference utilitarians count the killing of a being with a preference for continued life as worse than the killing of a being without any such preference. Self-conscious beings therefore are not mere receptacles for containing a certain quantity of pleasure, and are not replaceable.
To take the view that non-self-conscious beings are replaceable is not to say that their interests do not count. I have elsewhere argued that their interests do count. As long as a sentient being is conscious, it has an interest in experiencing as much pleasure and as little pain as possible. Sentience suffices to place a being within the sphere of equal consideration of interests; but it does not mean that the being has a personal interest in continuing to live. For a non-self-conscious being, death is the cessation of experiences, in much the same way that birth is the beginning of experiences. Death cannot be contrary to a preference for continued life, any more than birth could be in accordance with a preference for commencing life. To this extent, with non-self-conscious life, birth and death cancel each other out; whereas with self-conscious beings the fact that once self-conscious one may desire to continue living means that death inflicts a loss for which the birth of another is insufficient gain.
(Peter Singer, "Killing Humans and Killing Animals," Inquiry 22 [summer 1979]: 145-56, at 152 [endnote omitted])
Note from KBJ: Singer is making a distinction within the class of sentient beings. Those that are self-conscious are not replaceable, whereas those that are non-self-conscious are replaceable. Suppose pigs are non-self-conscious. Then painlessly killing a pig while replacing it with another, equally happy pig is not wrong. Suppose humans are self-conscious. Then painlessly killing a human (specifically: one who desires to continue living) while replacing it with another, equally happy human is wrong. Note that this distinction does not make Singer a speciesist, since it is not species that makes the difference. It is self-consciousness. While self-consciousness may be correlated with species, it is not identical to it. Self-consciousness is morally significant; species, like race or sex, is not.