It is said that animals cannot behave immorally because they are incapable of discerning right from wrong. But why is this relevant? Chimpanzees murder one another on occasion, for example. If murder is inherently wrong, what does it matter that the chimps don't know it? Surely, we wouldn't allow moral ignorance as an excuse when a human commits murder. (Not to mention the fact that chimpanzees probably shun other chimpanzees who've committed murder, so how can we really be sure they don't have any moral sensibilities?)
The only way I can think of this being relevant is that morality actually has nothing to do with the actions themselves, but rather has to do with how human beings relate to these actions. If murder were wrong because of features inherent in the act of murder, than chimpanzees who kill others would be just as morally guilty as humans who do so. Murder must be wrong because of features inherent to humans (as we are the only candidates for moral agency we know of), and the way we relate to murder. Ultimately, though, doesn't this mean that morality is entirely mutable, via thought or culture or even only through something radical (yet plausible) like genetic modification?