I am always surprised by the resistance which the classical definition of man as a rational animal stirs.
Rationality has two aspects, one concerning how I decide what to do, the other concerning how I recognize the way things are.
As to the first aspect: Humans do not always act for good reasons, but we always act for reasons. The other animals do not have reasons; they only have motives. To put it another way, they are incapable of reflection on their motives; for them, the impulse to do something is simply a given. Even when they are torn by conflicting impulses, they don’t ask themselves questions about them; they go the way the stronger one directs. Not even the most confused animal asks “what does all this mean?”
As to the second aspect: Humans may not know much, but knowledge is always an issue for us; for animals, it never is. However superior their sight or sense of smell may be to ours, they take in only facts, not truths. Incapable of reflection on the correspondence between thought and reality, they cannot grasp what truth is. They form habits, but not generalizations; they try to find things out (like where the mouse is), but not even the most curious cat longs to grasp the meaning of the whole. A creature could even be more clever than we are, in a certain sense smarter than we are, and yet not be rational.
The Psalmist bids the birds, the skies, and even the reptiles to praise God, and they do so, just by being what He has made. Yet we alone are capable of reflecting on that soaring hymn. The skies are not aware that they declare His glory; the bird does not know that it exults. Among all the animals, we are His only images, because we are the only mirrors.