Polar foxes have the warmest fur of all mammals. 70% of it consists of thick undercoat. It is the only wild dog to change fur colours twice a year. In winter, it is white as snow and in summer brown. This way, it is well camouflaged even in the tundra after the snow has melted.
Fun fact Polar foxes have thick fur even on their paws. It keeps them warm and provides secure footing.