Great, great stuff. But here goes some critique on stuff that popped:
The volumes on the torso, given the lightsource, aren't very consistent. The shadow on the ground indicates the sun is at an angle, but the shading on the torso seems to indicate a lightsource directly from the top. It'd be more interesting to follow that angled lightsource.
Also, i'd remove the waist tapering, and reduce the chest and shoulders width, to show a more rat-like anatomy and less human like, i don't think it's working very well as it is (what with that pouched belly going on, right under a very thin waist).
Other than that,s that's some bad ass rat yo, fo sho'.
Now with rough edit. Added some reflected lighting from the sand.