For the last week and a half, I have been doing intros between my existing trio of girls (One aged 1.5 years, two aged 8 months) and two new rats (both 8 weeks). I've been taking it slow - separate cages to get used to each others smells, then group intros in a neutral place, then group intros on a place the resident rats are used to playing, and swapping cages for half an hour to get used to scents - but for some reason my oldest girl seems to be behaving suddenly aggressive to one new rat in particular. Now, the rat she's behaving aggressively to does not provoke her whatsoever - in fact, she's probably the tiniest 8 week old rattie ever, less than half of my older girls' size. Furthermore, she is the gentlest creature - when interacting with the other rats she walks under them and never ever tries to dominate. Yet even when she's minding her own business, my older rat will walk up to her, hiss (sort of like a puffing of air noise), and lunge - not only will she pin her but tufts of fur will come out. It happens so quickly and there's a lot of squealing on the poor girls' part and I break it up because it seems far more aggressive than the forced grooming that is to be expected in intros. I have no idea why she's behaving like this. Further, the other youngster (who is way more mischievous and tends to try to dominate the older girls) doesn't seem to bug her at all. Do any of you have any ideas as to why she's behaving like this and what I can do to intervene? Thanks so much.