Any thing that strips guard hair from the animal, thins the fur. Brush is a constant. Yes, I imagine mating takes it's toll too, as does freezing to the snow. The rubbing from the snow is usually on top of the rump. I had a real problem with that in Alaskan fox. They would sit in snow, and leave hairs frozen in it when they stand up. In any case, yes, you can find individual animals that still have good fur in Feb. and March, but the majority will not grade well. Coyotes that spend most of their time in stubble, or alfalfa fields lose less than coyotes in brushy areas. It's simply wear and tear. That's why they need to grow new fur every year, to keep from going bald!