Hibernation doesn't happen on a whim. Throughout the warm season, animals prepare for it by eating a lot to build up reserves and by arranging their shelter.
Burrows, hollowed-out tree stumps, crevices in rocks, old abandoned houses... Hibernating animals sleep in a very peculiar way.
Their heart beats slow down, they barely breathe, and they become very cold.
The marmot's body temperature drops to 8 degrees. The hedgehog can go an hour without breathing...