Which of these animals hibernates through winter?

Bears are the famous answer and the technically contested one. True hibernators - ground squirrels, marmots, some bats - drop their body temperature almost to freezing and become genuinely unrousable. A bear's temperature falls only a few degrees and it can wake and defend itself immediately, which is why some biologists prefer to call it torpor or winter dormancy.

What a bear does is arguably more impressive. It spends months without eating, drinking, urinating or defecating, recycling its own urea into protein, and emerges having lost fat but very little muscle or bone - where a human immobilised that long would suffer serious wasting and osteoporosis. Why they do not is an active area of medical research. Females also give birth during it, and nurse cubs while asleep.

The extreme end of the phenomenon belongs elsewhere. Wood frogs in Alaska let themselves freeze solid - no heartbeat, no breathing, ice between the cells - protected by flooding their tissues with glucose as antifreeze, and thaw out in spring. And hibernation is not always about cold: some tropical species do the same thing through drought, which is called aestivation.