Why do we have a moral obligation to animals who do not understand moral concepts?

There are many human beings who do not have the ability to understand moral concepts. Examples are infants, the severely mentally retarded, and elderly people who's minds have deteriorated.

Nevertheless, we believe that we have an obligation to act morally towards these individuals, even though they themselves do not have the ability to make ethical decisions.

Incidentally, it is worth pointing out that human beings are not the only animals capable of understanding moral concepts.