The deer mouse, believed to be the most common mammal in North America, has a very different take on family values than its evolutionary sibling, the oldfield mouse.

Oldfield mice are monogamous. Fathers groom their young, keep them warm and ensure they don’t wander far from the nest. The deer mouse prefers the swinging lifestyle when it comes to sexual partners. It’s not unusual for the pups in a single litter to come from four different fathers. As for the deer mouse dads, they’re downright negligent. Nothing, it seemed, could bring out the warm and fuzzy dad behavior.