A common quip--even one that I have used in teaching--is that animals aren't moral agents. Squirrels and tigers aren't responsible for their actions in the way that we regard humans as responsible (and as we hold ourselves responsible). I generally say something like this in ethics classes when talking about the significance of morality for humans--it's part of what we take ourselves to be. (Sometimes the claim comes out in discussions of animal ethics, though I haven't done but small bits on it so far. That all will change soon, as I'm teaching classes on animal ethics next fall and spring!)
But is it true that no animals are moral agents? In Adam's Task, Vicki Hearne insists that the relationship between a dog and a trainer is a moral relationship, in which the dog must be held responsible for its actions (and in a much more structured way than the typical, "No, Fifi! Bad dog!"). For Hearne, the idea is that training only occurs by holding the dog responsible (she is no behaviorist), and the dog can learn what is expected of it. The result of this, on Hearne's view, is that the well-trained dog earns a kind of freedom. When Hearne tells Salty, "Salty, Ok!" that means Salty can hang loose, and do what she likes. But Salty knows that she is not free to dig holes in the yard or terrorize the kitty.
Now, one might say, but Salty isn't a "full-fledged" moral agent; she isn't responsible for the course of her life in the way we might say that of a person. And perhaps that's right. But if Hearne's description of the relationship between dog and trainer is right, we can't say there is nothing of moral agency in Salty's being either. (Hearne also has intriguing things to say about the problematic nature of authority which necessarily structures this relationship, but I'll leave that for another time.)
(Thanks to one of my mentors, Ed Minar, for bringing my attention to Hearne's work. If you're looking for a primer, she has a nice article in Harper's entitled, "What's Wrong With Animal Rights?" (September 1991). Preview here, but you have to pay.)