Rat Attack from Established Cagemate After Surgery--Help!

My poor girl Zelda just got back from the vets' after having two more mammary tumors removed. I kept her isolated for one day to see how she fared, and when she was perking up and eating again, I put her back with her 4 cagemates. They've lived mostly harmoniously for six months with no issues. She had surgery 3 months ago for tumors, and there were no problems (except the darn things grew back).

But yesterday, we heard terrible screaming, and found Zelda at the bottom of the cage with a huge gash opened between her shoulders and another 2 lacerations near it, about an inch long each. What happened?? I didn't think they were bite wounds, but could find no other reason for them. I took her to work with me today to keep an eye on her. When I got home, I put her back in the big cage. About a half-hour later, she started screaming again, and this time we found Maia attacking her. Maia is the newest addition, about four months ago. She is the most energetic and somewhat aggressive, starting fights with the other younger rats but never with Zelda!

Is it possible that Maia senses Zelda's injuries and is trying to kill her?? Zelda misses her sisters and needs to be with the pack, but I can't have her attacked again! I only have a tiny secondary cage and am hoping I won't have to buy a second big cage and maintain two colonies. Any advice is greatly appreciated.

Truthfully, the only thing I can think of is that her scent would be different because of staying at the vet/surgery/etc. Since she's gotten some pretty severe wounds, I would keep her separated until they heal. When you go to reintroduce her, do it out of the cage, under supervision. At the same time, SUPER CLEAN that cage down. Make it smell like nothing. Then try just dotting some vanilla essence on everyone's chests. This can confuse their sniffers.

Unfortunately, when a rat smells differently, some rats really don't like that or feels threaten by that. It's uncommon in established colonies but it does happen and I've noticed it seems worse with girls being the aggressor. It might be best to start intros all over again. I would take the newer girls out and starting like new once Zelda is completely healed.
The good news is that it's probably not a bite but instead, Maia tried to mount zelda to power groom and when Zelda protested, she got sliced in the process.

Well, I spent an hour this morning, observing the colony behavior. I put vanilla extract on all of them so they smelled the same. I've come to the conclusion that Maia is simply a terror.

Here's the colony: Zelda is the oldest, at around 24 months. She's had 2 tumor removal surgeries in the past 3 months. Noelle is age unknown but considered young. (I adopted her from my vet when I saw her in a cage with her 10 babies. Figured the 10 would find homes and she wouldn't.)

Selena and Athena were acquired in December when Zelda's cagemate Minnie passed away, so I figure they are about 7 months old.

Maia was rescued from pet store after employee told me she was "snake food." I think she must be about four months old.

Today, after the vanilla scenting, I put them all together again in the big cage. I saw right away Zelda's behaviour completely changed. She snuck up to the hammock, slithered in with Athena and stayed very, very still. Noelle and Selena were at the bottom of the cage, frantically grooming off the vanilla scent. Maia had a snack, then went down and mounted both Selena and Noelle aggressively, resulting in loud squeaks from both of them. It's almost like Maia is socially awkward and doesn't know how to approach a group without being inappropriate. She laid under Noelle and seemed to be asking for a groom, and when she wasn't given one, she got pissed off and bit her.

Then, Maia went up to the top and sniffed around in a frantic way. I noticed Zelda's eyes popped open and her ears were far back. She sleeps with her head hanging over the side of the hammock. Maia went underneath her, and sniffed her snout. Then, bam, she attacked Zelda from underneath her! What the heck??

I immediately grabbed Maia, shouted "NO" and put her in the small cage. Maybe this will sound weird to some, but she gave off a certain smell, not like she passed gas but something unpleasant I cannot name. Zelda immediately started looking and asking to come out of the big cage. I let her walk around the outside of the small cage so she could see that Maia was in it, and Maia rushed at her, scaring her and sending her running. This is just so out of hand!

How could an established colony get turned upside down like this? I don't want to keep Zelda isolated because it's better for her to be in the warmth and companionship of her sisters, better for healing from her wounds. I don't want to keep Maia separated either, as I feel badly for her, but I simply cannot let her keep terrorizing everyone, months after she was introduced and things were fine! She does have a torn-in-half ear from some recent scuffle, as she does start fights, but for the most part they were all fine.

I am at a loss as to what to do. I hope I don't sound like a crazy person with these observations, but I, like many of you, find rat behaviour fascinating and know they are very intelligent and complex beings.

Any advice is greatly appreciated.

Was Maia properly introduced when she came in the colony? Does she get along with any of them at all? Perhaps you could split your colony into two groups for now, and work on slow intros / "playdates" if Maia has someone she gets along with.
Some say spaying can help with aggressive female, is that a possibility?

I was wondering about that. I did find a vet who will spay, but it's $120 per rat, plus meds, which isn't doable for me right now. My own vet won't spay, because he says whether he does or not, they live to be about 2. Yes, she was introduced over several weeks and things were fine. She behaves in such a way that I wonder what's going on in her head. She is very in-your-face about things, afraid of nothing but our cats.

I guess I'm going to have to get another cage and put Maia and maybe Athena in it together.

I too would try a spay but it's definitely not a guarantee that it will change her.
I would also keep doing a time out cage for her. Make sure that time out cage is completely empty. No water, no food, no treats, no bedding, nothing. Put her in there every time she attacks any rat, keep her in there no longer than 15 minutes. Try that for a few days. If that doesn't help, then I would move Maia to her own cage for 2 weeks and start intros over again.

Hi everyone,

Thank you for your advice concerning Maia the Scrapper, as we call her. We did the cage timeouts and she seems to have reacclimated into the colony. She sometimes attacks a sister, but they seem to put her in her place.

I am strongly considering spaying her, just as much for the prevention of tumors (her oldest sister Zelda has had four surgeries for them and is not doing well) and I want to prevent it in the youngers if I can. But it's expensive to spay them so I will have to do it as budget allows.

Anyway, I posted separately about Zelda's health issues, but wanted to post a followup here and thank you for your responses.

I think it's a good idea to spay your girls, even if you do it slowly and when you can afford it. That would definitely be my game plan too.

If you have a good vet, then spays are a good idea for health reasons

You are fortunate, it is $200 a spay here.
I feel your pain, I have 4 girls that need spays