Full Idea
Man has duties only to men, ...since his duty to any other subject is moral constraint by that's subject's will. Hence the constraining (binding) subject must first be a person. Man can therefore have no duty to any beings other than men.
Gist of Idea
Men can only have duties to those who qualify as persons
Source
Immanuel Kant (Metaphysics of Morals II:Doctrine of Virtue [1797], 442 I.I)
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.237
A Reaction
This is good for illuminating why I am not a Kantian. It is not just that animals are ruled out - it is that whether you show respect depends on whether the recipient passes some test or other. Humans with brain damage may fail the test.