Bones are living tissue. Tissues continually break down and then replace themselves.

VOA: special.2010.08.31

Deep in oceans around the world lurks a type of worm without a mouth, anus or gut that makes its living by eating the bones of whales and other deceased sea creatures.

MSN: Zombie worm has no mouth �� but it feasts on whale bones

One of the risks of living in microgravity conditions is that bones tend to lose strength over time.

BBC: Shuttle Endeavour launches from Florida