David Hume

“ As animals are but little susceptible either of the pleasures or pains of the imagination, they can judge of objects only by the sensible good or evil, which they produce, and from that must regulate their affections towards them. Accordingly we find, that by benefits or injuries we produce their love or hatred; and that by feeding and cherishing any animal, we quickly acquire his affections; as by beating and abusing him we never fail to draw on us his enmity and ill-will.
Love in beasts is not caus'd so much by relation, as in our species ”

“ But to pass from the passions of love and hatred, and from their mixtures and compositions, as they appear in man, to the same affections, as they display themselves in brutes; we may observe, not only that love and hatred are common to the whole sensitive creation, but likewise that their causes, as above-explain'd, are of so simple a nature, that they may easily be suppos'd to operate on mere animals. There is no force of reflection or penetration requir'd. Every thing is conducted by springs and principles, which are not peculiar to man, or any one species of animals. ”

“ Thus acquaintance, which has the same effect as relation, always produces love in animals either to men or to each other. For the same reason any likeness among them is the source of affection. An ox confin'd to a park with horses, will naturally join their company, if I may so speak, but always leaves it to enjoy that of his own species, where he has the choice of both. ”