Like many other mammals, deer physically prepare for the winter by better insulating their bodies. In the fall, deer gradually trade their summer hair coat for a winter one, which consists of thicker, longer, and darker hairs called guard hairs, while also growing in a much thicker undercoat.
This winter coat absorbs more sunlight and traps more body heat than the summer coat, and provides an extraordinary amount of protection from the cold. Deer also have oil-producing glands in their skin that help make their hair water repellent, which is especially valuable in the snow. For further insulation, their bodies also begin to retain more fat in layers during the fall.
Deer also alter their behavior to survive the winter. They are generally less active, sometimes dropping their metabolism by half, which allows them to save energy and eat less. Deer may physically hunker down during particularly harsh weather—not moving for days, even to eat— which is made possible by relying on their fat stores.