Sure, those are traits we use to distinguish different rights among sentient (but not sapient) animals--but it's all rather arbitrary. How do we define "suffer

Sure, those are traits we use to distinguish different rights among sentient (but not sapient) animals--but it's all rather arbitrary. How do we define "suffering"? Is it the same emotional experiences we call "fear" and "sadness", i.e., the ability to have the concept "I am afraid/sad"? That means the animal needs to at least have a primitive form of understanding and introspection to qualify (e.g., great apes, chimps, bonobos). If we go to the other extreme, we must admit we don't have a reliable method for communicating with animals (famous "dog whisperers" aside). We can only infer "suffering" is taking place when the animal reacts negatively to a negative external stimulus. But bacteria respond negatively to a negative external stimulus as well. Sure, there's mind-bogglingly more complexity in a mammalian brain than a bacterium, but if we reduce "suffering" to the observation of a negative reaction to a negative external stimulus, we must say even single cell organisms are "suffering." Hard to strike the right balance.