Air trapped in the fur of the caribous coat functions as _.

Body heat because To keep the heat in, caribou have two layers of fur covering their bodies. They have a fine a fine crinkly under-fur and a thick coat of guard hairs on top. Guard hairs are hollow like straws. The air trapped inside the hollow hairs act as insulation to keep in the caribou's body heat.

So the answer is Body Heat

Answer is Body Heat