The crux of this issue has as much to do with humans as it does with animals. When humans act morally, what is it we are doing? Traditionally, the philosopher’s answer has been an intellectualist one: acting morally requires the ability to think about what we are doing, to evaluate our reasons in the light of moral principles. But there is another tradition, associated with the philosopher David Hume and developed later by Charles Darwin, that understands morality as a far more basic part of our nature — a part of us that is as much animal as it is intellectual. On this ‘sentimentalist’ account of morality, our natural sentiments — the empathy and sympathy we have for those around us — are basic components of our biological nature. Our morality is rooted in our biology rather than our intellect.
If this is true, then the reasons for thinking that animals cannot act morally dissolve before our eyes. What is left is a new understanding of what we are doing when we act morally and, to that extent, the sorts of beings we are. Those beings are, perhaps, just a little more biological and a little less intellectual, a little more animal and a little less spiritual, than we once thought.