5. Keeping warm.
Wildlife in Winter
Animals use different strategies to keep warm. Some find shivering useful to generate heat, but it helps if you have a warm coat to begin with. Deer, who shed twice a year, put on a warm coat in fall. The outer hairs are hollow to provide insulation, and the inner coat is a thick fur, five times denser than their outer coat (and four times denser than sheep’s wool). In summer, they lose the fur, and the outer coat is made up of solid hair shafts. Birds can fluff up their feathers, to stay warm by trapping air inside. Some frogs produce a kind of antifreeze, which allows them to become frog-cicles and then revive in springtime.