Chipmunks are one of a number of rodents which use cheek pouches to carry gathered food.  Eastern Chipmunks store food in their burrows, and most of their activity during the spring, summer and fall is devoted to foraging for and storing grain, seeds, nuts and acorns for winter.  Though they do not actually hibernate, Eastern Chipmunks do something very similar, staying entirely in their burrows and going in and out of a hibernation-like state called "torpor" throughout the winter.  They do not put on fat stores for the winter, but instead rely on the food they have stored in their burrow.