Mice can scale walls and climb down the rope that leads to your hanging food bag. Put in their path an obstacle they can't get around a tin can or plastic lid usually works. Cut a small hole in the bottom of an empty can or in a plastic coffee-can lid and thread string or rope through the hole. Tie a knot in the string so the can or lid rests on the knot. Attach your food bag to a stick on the bottom of the string. Most mice cannot get around the lid or can. You should also leave zippers and pockets on your backpack open so the critters can explore without chewing holes through your pack.