They do not enter winter with a thick layer of body fat, as do true hibernators. In late October or early November, they go underground to live on stored food until spring. Wintering behavior varies. Another researcher cited 30 percent as becoming torpid and classified 60 percent as non-hibernating. In two studies, inactive periods lasted from 1-8 days.
Chipmunks plug their den entrances when they go underground.
They sleep much of the winter, waking periodically to eat. On still, sunny days they may leave the den briefly. Some emerge to breed in late February or early March, but snow and cold may drive them back into dormancy. By late March, breeding is in full swing. A female may bear two litters per year, the second in late July or early August.