oxytocin surge at dusk
Gray wolves and prairie voles...
Prairie voles are rare among mice for their "faithful marriages" (pair bonding, mongamy, and joint rasing of the young). Other voles and mice exhibit none of those traits. Tom Insel has done some fascinating experiments to genetically regulate the number of oxytocin and vasopressin receptors (which link to the D2 dopamine receptors associated with drug addiction) in the vole brain. Mating causes a release of both hormones, but the right receptors have to be in the brain for the pair-bonding to follow.
In short, Tom was able to switch monogamy on and off. Monogamy may boil down to the length of a DNA sequence in a promoter switch at the front of a particular receptor gene.