hi!! just wanted to let u know we had the same problem with the female rat we bought a few months back... we had a male before we bought her, but he had passed due to tumors. we had no problems with aggression at first, but after about 2 weeks of having her, she started to nip and scratch a lot... well.. we let her alone and tried to avoid too much disturbance around the cage, because we were worried it might be due to the fact we also have three cats. none of that seemed to make a difference. then about 4 weeks later she gave birth to 11 little ones... she was already pregnant when we purchased her apparantly and she did not appear to be pregnant to the naked eye.... after the birth and since we've found homes for all of her offspring she is a very social little thing and even sits on the cats and sleeps... don't know if that helps at all, but i thought i'd share!!
good luck!!
