One day last week I let my cat out into the backyard and a few seconds later followed her. As soon as I stepped on the deck, I heard the chirp-like sounds eastern gray squirrels (Sciurus carolinensis) seem to make when they spot a predator, in this case, probably the cat. I looked up at the nearby pine tree and saw a squirrel jumping from one branch to another and, at the same time, noticed a large white object falling to the ground.
It turned out to be the pelvic bone of a deer. No, the squirrel hadn't dragged a deer up the tree for dinner; it was one of the many deer bones that we had once collected in the nearby woods and now are scattered throughout our yard.
The close examination of the bone solved a mystery that I had first written about in this post: who's been gnawing on our deer bones? The pelvic bone had numerous distinct scratch marks along its edges. These are probably the squirrel's teeth marks.
So the squirrels are the bone eaters. But, why do they do that? A quick look at The Encyclopedia of Mammals (David Macdonald, ed., 1984) suggested one possible answer. As in other rodents (for example, beavers), squirrels' incisor teeth grow continuously. So they have to keep chewing on hard objects, nuts and the like, to wear their teeth down lest they get too long to fit into their mouths.
Another possibility that I can think of is that squirrels satisfy their calcium requirement by gnawing on bones.
Here is another detail from the same bone.