This word, which probably originally implied belief in another person, (faith+full) is decidedly a human characteristic, since few animals, other than pet dogs, display it. As regards marital fidelity, with the U.S. divorce rate at 50%, it is not so human after all. But faithfulness can also be for a friend, to an idea or ideal.
It transcends nature. However, Canada geese mate for life, unlike most other birds, and both parents share in raising the young. With most mammals, however, this task is borne by the female, except with humans. Faithfulness, with them, is classified as a virtue, a strength, and greatly admired among the civilized.
It is sometimes unquestioning. In Lucerne, Switzerland, there is a huge stone monument to the Swiss Guard of Louis XVI, men in his employ, who died almost to the man defending him from the French Revolutionaries. That’s faithfulness.