The vultures could not have eaten up both body and bones.

There is no skeleton, no remains.

His fellow fugitive has gone off or been taken.

Whither?
While asking the question Wilder sets about the right way to answer it.
As a skilled tracker he begins by examining the signs that should put him on the trace of his missing companion.

At a glance he perceives the prints of a horse's hoof, and sees they are those of one unshod.