If for humans fidelity has a social and moral dimension, it is not the same with our animal friends: indeed, for them, the survival of the species prevails. But is it true for all animals?
First, it is more monogamy than fidelity when it comes to animals, that is to say two animals remaining together for a certain period of time, usually because of their cubs. This monogamous system is not advantageous for the survival of the species, nor for males who need to reproduce frequently. But even for these monogamous pairs, "adultery" is not rare!
But there are some examples of true couples animals, such as the Gibbon. This primate family lives with its cubs, and it is the mother who is the head of the group! The Swan and the black vulture are also monogamous, as Albatros or Doves. Indeed, there are more "faithful couples" within birds!
So there is finally some faithful animals: indeed, their instinct is to contribute to the continuity and survival of the species, humans, themselves, choose their partners consciously and not instinctively.