u/Inevitable-While-577

Please note: This is not about nibbling, biting from poor eyesight, thinking that hands are food. It's not only about biting when picked up. It's not about hormonal aggression. The rat has clearly aggressive body language and actively attacks/hunts me.

TL,DL: rat too old to unlearn aggressive behaviour - need a way to get her to relax, at least enough to not actively attack. Most important parts of post are bold.

Long story short, I recently adopted 2 new rats from a shelter with the intent of introducing them to my 2 girls (because sadly after several deaths, I had only 2 left). They're currently in a quarantine cage in a separate room.

They're female, 1.5 years old. Haven't been handled much and afaik always lived as a pair. They don't know how to lick yoghurt from a spoon or finger, they seem unfamiliar with a lot of things. I let them free roam daily in the room, and they seem to enjoy it.

One of them is very shy, easily startled, can't be picked up, typical rescue rat behaviour. (She'll sometimes bite my hands softly in the typical "is this food or body part" way, which is fine, I know how to deal with that. I've had several like her before.)

The other one however is seriously aggressive in a way I've never experienced. She's not shy. She'll rush out of the cage as soon as I open it. She climbs on me since day 1, uses my body as a bridge. She acts confident but her fur puffs up as soon as she touches my body. She takes treats from my hands but her fur puffs up. She has even side walked (crab walked) me just sitting on the floor and handing her a treat. She chases and attacks my feet when I walk so I need to wear shoes. She hates being picked up, but she lets it happen more than her sister does (doesn't run away). Once she bit my finger and drew blood.

She'll climb up my leg or arm (with puffed up fur!) and all the way up to my shoulder. Once, she was on my arm, I was facing her and talking to her, she was completely still then suddenly darted to attack the collar of my hoodie. I'm lucky she didn't bite a few centimetres above, on my bare neck. I am now scared of her and don't want to let her on my shoulder or near my face.

My concern is that since she's pretty old, it's probably learned behaviour, not hormonal, so spaying wouldn't help and it's possibly too late for her to unlearn. I have no idea how they were treated but it must have done a lot of damage.

I haven't started introductions. I fear that since they've never lived in a group, she might turn out equally aggressive towards my rats, but I don't know that yet. If I'm lucky, her behaviour is only towards humans. I've had many rescues and I don't need them to be cuddly with me or very easy to handle. I'm fine as long as they all get along. But in her case, I want to at least stop her from actively attacking me.

So here's my actual question: how do I teach her not to actively attack me? I feel there must be two approaches: either try to "dominate" her, basically force her into relaxing with me, kind of like a dominant rat would do (force grooming, etc.); or the contrary, showing her that she won and I'm "submitting", though I have no idea how one would translate the latter into rat language. The former seems more feasable but will this really serve to calm her down or would it just traumatise her more?

She's an extreme case because she's fairly old. I feel at this point, the gentle method (treats, sitting with her calmy, letting her climb on me) will NOT work. And I can't limit interaction either (as one would with a skittish rat) since she actively approaches me.

If anyone has had a similar case, I would really appreciate some insight.