My argument for a long time was that animals have no ego, therefore death is insignificant to them. Animals are incapable of forming abstract concepts, of thinking about the future or the past in concrete terms, of forming feelings and emotions.
However, there's plenty of evidence to suggest that at least the more intelligent mammals, such as whales and dolphins, as well as apes, do have an ego. Plenty of people argue that it's unethical to kill whales and dolphins because they do have an ego and can form abstract thoughts.
So apparently whales are off the table, but cows are fine? Where is that line drawn? I don't have that answer, so I've gone off eating meat in general. The concept of killing a living, possibly thinking being in order to eat it seems a bit fucked up to me these days. I can't say with certainty if/what animals do have an ego, or if they are capable of abstract thoughts, but it for me it seems safer to assume they do and avoid eating them.