"their own crude writing" [Digest urth.v006.n028]

With respect to rats writing, I read that as saying that rats fouled the books with their waste and left footprints, etc. These tracks and evidences are a kind of writing in that one can "read" them, gleaning details of the rat's life. In this sense, a footprint means "I (a rat) was here". Check out semiotics for careful definitions of the difference between a footprint and a drawing of a footprint and so on. -John