What did Sam mean when he said that "seeing The Baron changing his mantle for winter awoke the first fears in me"?

It's chapter 13 of My Side of the Mountain and fall has almost arrived. As Sam looks about him, he notices that nature is changing to accommodate the new season. Leaves are changing color, the harvest is ready to be gathered in, and the animals are preparing to migrate to warmer climes or hibernate for the winter.
One such animal is Baron the weasel. His mantle, or fur, is changing from his light summer coat to his heavy winter coat. This make Sam fearful for the first time as he realizes that there's not long to go before the harsh winter months set in. So far things have been going quite well for Sam. Since he first rocked up in the Catskills he's gradually learned to adapt to life in the wilderness. But spending his time outdoors in winter will be a whole new challenge completely, much more difficult that anything he's experienced so far. And the prospect makes him more than a little frightened.