I dont know if the rats know the snake is a threat or not, I usually find un-eaten rats cowering in a corner looking scared, but I am unsure if that is due to me ripping them out of their nice warm pile of other rats and throwing them into a new place or not. I find that mice will run around the bins more often than rats, which usually "hide" in the corners, usually between the water dish and the bin. I would say when you switch to rats, just keep an eye out, and as someone else posted make sure they are hydrated and well fed. The only time I have ever observed a rat or mouse, any rodent, biting one of my snakes is while they were constricting it, and I can usually get a pen or feeding tongs in the teeth, sometimes not, but I have yet to see any injuries due to it.