Caribou and their relative, the reindeer, also live in the Arctic. The caribou are now found mostly in North America, and they are larger than the reindeer. Large herds feed on the grass and lichens. When fall comes, they move south to winter feeding grounds. They dig through the snow with their sharp hoofs for food. Their hollow fur creates a cushion of air around them that helps them to stay warm. In the spring they return north to the summer pastures, where their calves are born. An hour or so after being born, the calves are able to follow the herd toward the Arctic Ocean. Most of the world's reindeer live in Europe and Asia. Some of these reindeer are not wild. They have been tamed by herdsmen who protect them from wolves and lead the migrations each spring and fall. Also, the musk ox, which looks like a shaggy buffalo, lives in the Arctic.