I have two 6-wk-old female rats. They are sisters (litter mates) but have had about a week apart and were only reunited 2 days ago. Sparkle has been with me for over a week. Tink has only lived here since Wednesday. They often seem to be furiously nibbling each others backs - is that just grooming? And sometimes they tear about and look as though they are fighting, each trying to get on top of the other, and I hear the odd little squeak. They don't have any injuries. It is pretty frantic stuff. It sometimes happens when I have had one out but not the other, and then I put them back together. It has also happened when I put them both in a (very big) new box while I cleaned out their cage. It doesn't happen very often. Mostly they are calm and relaxed and potter around together and sleep curled up together. Are they playing or fighting or establishing dominence or something else?