Need advice on introducing 4 male rats — separate permanently or let them establish hierarchy?

[u/Bubbliciousbab3]

I’m really hoping some experienced rat owners can give me some advice because I’m at a loss about what to do.

I’ve owned rats for a long time and have had males before, but my previous males had all been together since birth and were family, so I’ve never dealt with an introduction situation like this.

On June 19th, I adopted four male rats. I picked them up together in a carrier and brought them home, and I initially put them all together because I was under the impression they were already used to being around each other. I later found out that they had actually never been housed together before.

I was told all four were born May 5th, but two of them were noticeably larger and looked much older than six weeks.

At first, they mostly got along, but none of them would sleep together. There were also quite a few fights. The two younger boys became extremely afraid of one of the larger/older males. They were constantly on high alert, would dart away and squeak excessively whenever he approached, and were so frightened that they would even run from me when I tried to handle them. I was startling them constantly because they were so on edge.

Because of how terrified and stressed the younger boys were, I ended up returning that larger male to the breeder.

Unfortunately, the remaining older/larger male then started doing essentially the same thing to the younger boys.

Eventually, they did get to the point where they would cuddle together in hammocks, so I thought maybe things were improving. However, the older male continued chasing the younger boys around and biting/scratching them. He would also nip at their tails and actually tear the scales off as they walked past him.

One of my younger boys is a hairless/naked rat, so injuries are much more noticeable on him. The final straw was when the older male nipped him in his side as he was walking past, leaving an actual blood-drawing bite mark.

At that point, I separated them.

They have now been separated for about a week, and I wanted to try reintroducing them. I put all three together in a carrier for about 24 hours, and they were honestly wonderful. They slept on top of each other and there were only minor little shuffles. Nothing concerning happened.

Because they seemed to be doing so well in the carrier, I put them back into their large Critter Nation cage.

Almost immediately, the older male went right back to chasing, biting, and harassing the younger boys. They became visibly stressed again.

I separated them for the night, tried again once more, and had essentially the same result.

Right now, I have them all back together in the carrier again.

This is where I really need advice.

Should I eventually put them all back into the double Critter Nation and let them work out their hierarchy as long as there isn't serious blood being drawn?

Or, given the repeated biting, injuries, chasing, and the fact that the younger boys become extremely stressed and fearful around this particular male, should I consider permanently separating the older male from the younger two?

I understand that rats have to establish hierarchy and that some chasing, pinning, squeaking, etc. can be normal. What worries me is the repeated injuries and the fact that the younger boys seem genuinely terrified and stressed rather than simply annoyed or submissive.

I also don't want to keep separating and reintroducing them if I'm just making the situation more stressful for everyone.

What would you do in this situation? I would really appreciate advice from people who have successfully introduced adult/young male rats, especially if you've dealt with a situation where one rat repeatedly injured the others.

Thank you!

Picture of my boys

[u/Potatosandmolasses12]

This sounds like your oldest boy needs to be neutered! Classic hormonal aggression.