Singer, Animals, and Pain.

In chapter four of the reader, Peter Singer talks about how animals, as well as humans, feel pain. Although we do not directly experience the pain of another being, we know they are in pain by the way they respond. In theory, we also would never wish pain upon others because we know what it feels like to be in agony. It wouldn't be right to want others to feel that way. Singer states that the vertebrates and nervous systems of most species (especially birds and mammals) are very similar. This supports that animals must feel the same way we do when we experience something that hurts and is painful. This is reason why some vegetarians do not eat meat; they do not want pain inflicted on any species since that pain is not being inflected on us. This is like the golden rule; "Do unto others as you would have them do unto you." This is partly why I do not eat meat. Animals aren't going out of their way to grill my meat so why do I have the power to grill theirs?

Pain is defined by the International Association for the Study of Pain (IASP) as “An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”. According to this definition, the only animals capable of feeling pain are those that experience fear, anxiety, distress and terror, similar to what humans feel. Some studies have shown that all animals (and maybe even plants) feel pain of one degree or another even if humans may not be able to tell. However, many people believe because we do not know that some animals (like fish or shrimp) actually feel pain, this is justification for killing and eating them. Also, some vegetarians choose to eat fish but no other animal and when I tell people I am a vegetarian, they ask "Do you eat fish?" Of course I don't eat fish, they are animals too and should be protected, in my opinion. Why do the fish get left out?

My question to you is: If an animal does not feel pain when being killed for food or otherwise, does this mean the killing can be justified? Also, if an animal truly does not feel pain, does this mean that that animal has less rights than other animals?