It should be said that friendship and enmity are found in brute animals. The reason for this is that friendship consists in the perception of what is agreeable and enmity in the perception of what is harmful. But these are found in brute animals, and therefore, etc. For birds perceive that a seed is agreeable to them, and a sheep or lamb perceives that a wolf is harmful to it but that a human or a shepherd is a friend.
St. Albert the Great, Questions Concerning Aristotle's On Animals, Book VIII, q. 1 (Resnick & Kitchell, trs.). He later argues that this is universal to all animals, because even immobile animals will flow over something they find agreeable and recede from something they find harmful. The actually interesting thing here, I think, is the conception of friendship as an agreement between two things, at least one of which is capable of perceiving the agreement, insofar as it is perceived. Not all of these agreements are of the same sort, of course, and so there are many kinds and levels of friendship. Each kind of animal (including ourselves, of course) is capable of some kind of friendship, and what we would call higher animals (including ourselves, of course) are capable of many kinds of friendship, depending on their powers of perception. But for St. Albert, animal life is by its very nature a life that involves interacting with the world in terms of the friendly and the unfriendly (or amiable and hostile), where this is not an anthropomorphism but a more general account of interaction with the world in which 'friendly' and 'unfriendly' in the sense we humans usually recognize is merely one particular kind of friendliness and unfriendliness.
