Not knowing anything about rats, but

I think my N800 would definitely react to rats walking on it unless they were good at distributing their weight. Would probably react to rats leaning against it with paws, if they do such a thing. Not entirely convinced though. The bigger contact surface, the more pressure required before it registers. That's why the stylus is pointy.

Keep in mind that the reported touch point will be the average of the pressure exerted on the screen.

There seems to be huge variation in the touch sensitivity between individual N8x0 units too...