Correct me if I am wrong, but I believe porcupines will also eat sheds, I suppose the size of the critter working on the shed will have something to do with how fast that particular shed disappears.

I have found sheds with blood still on them with significant chew marks on them, and I have found sheds that are green with age, with no marks on them whatsoever.

It all depends on what sort of wildlife is in the area and how lucky they are in finding these calcium treasures.