Frogs have long been considered among the most promiscuous creatures. But one species of frog, the Ranitomeya imitator, native to Peru, were discovered to remain monogamous after a series of paternity tests were conducted. The reason for this has to do with how the frogs raise their offspring. After the female lays her eggs on a leaf, the male collects the individual tadpoles as they emerge, moving each to its own private reservoir of water that's pooled in the crevices of leaves, high above the forest floor. When the tadpoles get hungry, the male informs his mate, who deposits an unfertilized egg into each pool for her growing young to eat. Researchers performed genetic tests on twelve of these frogs and found that eleven were completely loyal to their partner. Just one frog, a male, was observed 'cheating' on his mate--though it's uncertain whether or not the relationship was 'open'.
This unique cooperation between the male and the female from for the benefit of their offspring, researchers say, is what makes them so inseparable.