Muskrats do not hibernate. In frozen marshes they maintain openings by pushing vegetation up through cracks in the ice, building small domed chambers known as push-ups that stay open and give access to a breathing and feeding space away from the lodge.
Between push-ups they use trapped air. A muskrat releases an exhaled bubble that adheres to the underside of the ice; oxygen diffuses into it from the surrounding water while carbon dioxide diffuses out, and the animal re-inhales it on the return trip. The same behaviour has been documented in beavers. It extends underwater range beyond what a single breath allows. Muskrats can also remain submerged for well over ten minutes, aided by tolerance of elevated blood carbon dioxide and by reduced heart rate.
Despite the name and the rat-like tail, the muskrat is not a rat. It belongs to the subfamily Arvicolinae, the voles and lemmings, and its closest relatives are other voles and lemmings rather than any Rattus species.