Genetic tests have revealed that male and females of Ranitomeya imitator remain utterly faithful to each other. More surprising is the discovery that just one thing - the size of the pools of water in which they lay their tadpoles - prevents the frogs straying. That constitutes the best evidence yet documented that monogamy can have a single cause.
A Key Ecological Trait Drove the Evolution of Biparental Care and Monogamy in an Amphibian. Jason L. Brown, Victor Morales, Kyle Summers. April 2010.