Well, there is just no question about it.  Mountain Men, Plainsmen, and their ilk were rugged self sustaining and independent individuals.  However, they got as cold as the next person.  They felt serious discomfort in the same way we all can feel it.  To be sure, these intrepid trappers liked to work into the season, late fall and early winter, to hunt and trap the very best paying pelts when the animals naturally put on the heaviest fur.  But in spite of the wide range of education found among them, they were not stupid.  What good is prime fur when you are stone dead from exposure and frozen, literally, to the bone?  So by a special sense of timing, they drifted back to lower climes and wintered, refitted, and put on some fat for the coming season.  Those who didn't disappeared.