It is of great consequence to have previously determined the concept that one wants to elucidate through observation before questioning experience ab… - Immanuel Kant

Thus our duties to animals are indirectly duties to humanity.

[T]he first characteristic of the human species is man's ability, as a rational being, to establish character for himself, as well as for the society into which nature has placed him. This ability, however, presupposes an already favorable natural predisposition and an inclination to the good in man, because the evil is really without character (since it is at odds with itself, and since it does not tolerate any lasting principle within itself)

A metaphysics of morals is therefore indispensably necessary, not merely because of a motive to speculation — for investigating the source of the practical basic principles that lie a priori in our reason — but also because morals themselves remain subject to all sorts of corruption as long as we are without that clue and supreme norm by which to appraise them correctly...