why is buck surprised when he is not allowed to sleep in the tent ?

Buck has always been afforded shelter and warmth. When he tries to sleep that night, he is unable to find warmth. After wandering around the camp, he decides to see how the team is making out. He comes across Billie buried in the snow, proceeds to make his own hole and fall asleep instantly. When he awakes, he instinctively forces his way out of the snow that has gathered over and around him. Without realizing it, he has begun to draw on the ancestral knowledge that is his birthright.