No success with terrified rat for months (terrified both for people and other rats)

Hi everyone,

This is the first time posting on the forum, it has been extremely helpful to read threads after threads of people facing similar issues as me and also reading the success stories. I am currently at a loss of what I should do and I hope that someone with similar experiences would be able to help me.

I have a colony of four rats (all male, un-neutered, all 1 year+) who come from various backgrounds, pet store and breeders, who all had their own little issues but nothing major. I handled everything pretty well in the past years and they're a happy family.

A few months ago I decided that I had enough time and space to see if I could adopt another rat. I adopted one (male, un-neutered, 9 months) from a random person on the internet without much consideration what living conditions the rat might've been living in. When I picked it up there were a lot of red flags that I did not take into account. Something I learned from.

Something I always did first was to quarantine the rat for 2-3 weeks and slowly build up trust. I handled him like my previous boys and realised that something was different with this rat. First when he would see my fingers, by grabbing him or giving him a treat, he would nib and bite hard enough for me to bleed. Nothing too deep though. I figured, and asked online, that this was purely out of fear. He never fluffed up in anyway for example.

Secondly it turned out he was coughing and I found out that he had scabs all across his body. I brought him to the vet and it turned out that he had mites. He got antibiotics for the coughing and anti-mite medicine. It took another few weeks until most health issues seemed to be fading away.

I read online that rats can be more fearful or aggressive if they're not feeling healthy. I figured this was the case but continuing the trust training did not really help. He continued to bite and I felt that with every bite we were moving backwards.

After losing much hope I turned to the internet and people said that it might make sense to introduce him to the others. If he would be fine with the other rats I could more easily build up trust by him seeing that the other rats were not fearful of me.

I started with the neutral place method (bath-tub introductions) as that helped me in the past with the other rats. I tried various options with him only introduced with the most dominant rat, other times with them all together. In all cases he was continuously and literally terrified of the other rats. He would scream and stand up, showing teeth, when they would get too close. He never fluffed up, therefore I feel it's not aggression per-se, but rather pure fear.

Again losing hope I read more about other introduction methods and the carrier method seemed to be the most promising. I figured that if he had no place to run, he would have to be fine with the others eventually.

It has now been 3 days since I started the carrier method. I started last Friday (Currently Monday) and put them in a small carrier with limited space and wooden bedding. Enough breathing room but little space between them. This seemed to go okay. A lot of screaming from the new rat. A lot of showing teeth from each other. He seemed in some situations to be cornered by two other rats. Other times everyone would raise their hands standing in a circle. It seemed cruel but it also seemed normal from what I read on the internet.

After 24 hours it seemed the screaming got less. I decided after 1 hour of silence in the early afternoon on Saturday that we could move to a slightly larger cage (with wooden bedding) which did not go very well I think. All my existing rats were huddled up in one corner while the new one was in the other. It seemed that by using the slightly larger cage they would not really have to be engaged with each other. I decided that they had to go back in the smaller cage. Another 24 hours passed.

Early Sunday morning I had heard no screaming and it seemed to go very well. I moved them up to the slightly larger cage which went okay. I left them alone for a few hours and when I came back I saw the new rat sleeping against one of the more dominant ones. I was really excited to see the progression.

This is where I might have made the mistake. I decided that it would be time, after 6 hours, to move to an even larger cage. This was a cat transport box, nothing too large. That also went well as this was the first time the new rat did not scream when I put him in. This was also the first time that I used fleece instead. I thought that it might be nice for them to hide somewhere.

They did not seem to hide and instead lay on top of the fleece. The new rat was sitting next to the door of the transport box and everything seemed to be quiet. One of the dominant rats went over and laid to the opposite side with his head, slightly sleeping. I saw as the new rat slowly approached his head, no fluffed hair or anything, and in a second trying to snatch the dominant rat by its head. He jumped up and a large fight bursted out, luckily no blood.

As I saw this happen I decided to downgrade their box to the smallest cage which I used in the beginning last night (Sunday). They have slept in that cage for the rest of the night until today. It seemed very quiet so I moved them up, once again, in the slightly larger cage this morning. After some boxing and rat-balling it (a slight nib on it's leg, nothing serious I would say) it seems to be very quiet and the new rat seems to lie down with the more dominant rat the last 30 minutes.

I would love to hear people's opinions to understand if what I am doing is okay. I also feel like I am losing hope as this is taking an extremely long time. Around me I have friends telling me that I should stop. They say it is cruel. Others say that I should accept the fact that this could be a solitary rat, that maybe it wants to be alone. Has anyone had carrier method introductions take this long? Is it fine to keep them confined in such a small space for such lengths of time, or should I give them breaks in between, back to their original cages? Is this behaviour considered aggression?

The last resort I also read is that I could neuter him. I don't know if this would help anything though, as I don't know if the fear is hormonal or not.

Sorry for the long post. Any information related to anything that I mentioned would be really helpful... I thank you in advance for reading my post! If anyone needs more information I am happy to provide.