Most mammals in winter either hibernate or go into a reduced activity mode called “torpor”. Their body temperatures lower and they require less energy to maintain warmth. Animals that go into this state generally have smaller extremities, shorter ears and compact bodies—all to help minimize heat loss. They don’t “sleep,” they just tone down all their non-essential heat-loss activities. Some animals that experience daily torpor include birds and some mammals, including many marsupials, rodents, bats and even some wild cats and bears.