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Pet Rats/Aggressive Boy


QUESTION: My little boy is becoming aggressive :(

He is 3 months old, and he was never at a pet store or rescue. I've been his only home since he was weaned.

His brother is as sweet as can be. He used to be, but now he's getting more aggressive..everyday.

It started off by him "biting" me when he went through his "teenage" stage. He didn't draw blood.

Now, me and my mom are the only ones who can pick him up without him biting.

He's bitten my friend which he's known since his birth (She raised him)
He's also bitten my friend who is over everyday, and handles him a lot. He drew blood on her.

Neutering costs $165! If I have to, I will get him neutered..But I was wondering if there was any other solution?

Also, if I do get him neutered, would it be okay to put him with my other boy who wouldn't be getting neutered?

ANSWER: You didn't mention if he is also showing any aggression toward his brother.  

Sometimes, male rats become aggressive towards humans and/or other rats at some point between 3-12 months of age, it is most common at 4-5 months.  This is because the level of male hormones increases at this age when they become mature.  Most male pet rats do not show aggression despite the hormone increase but in some rats, the hormone level is higher and those are the ones that show aggression.

Monitor his behavior to his brother.  Is he chasing him, nipping him on the rear end or back, or over-grooming him to the point of causing him to squeak a lot?

You also didn't say outright if this boy came from a breeder, but it sounds like he did.  If so, there are some breeders that are not careful to breed out negative behavior in their lines.  So even if this boy was handled from birth by the breeder, and after that by you and your friends, if he comes from a line of agressive rats, then this begins to show up at the age I specified above.  If his brother is from the same litter, there is a chance you may begin to see similar behavior in him as well around this age.

I completely understand that you don't want yourself or your friends to be in danger of getting bitten, but even worse would be aggression including biting to his brother.  Again, genetics play a big part in his behavior and there would be little you could do to train him differently if so.  I suggest waiting another week or two and watching him closely, with hopes that this aggression might just be a "phase" and will subside.  

However, chances are that it won't and the only way to reduce the male hormone levels is  unfortunately to have him neutered.  If you do procede with neutering, keep in mind that the hormone level will take several weeks to drop to the new lower level so you may continue to see some aggression until then.  Don't worry at all about putting him back in the cage with his brother.  They should continue to get along, perhaps even better.

I hope this was helpful to you and please let me know if you have any follow up questions.

---------- FOLLOW-UP ----------

QUESTION: He actually wasn't from a breeder. My friend bought a girl from a petstore, who had a litter of 5. I took the two boys.

He does chase his brother around the room. But he's done that since he was little. And I have noticed that whenever he grooms his brother, there is squeaking, but he hasn't shown "aggression" to his brother at all.

Well that may explain it then.  Most pet store rats are bred in rat mills/farms for the primary purpose of selling to pet stores as snake food.  The sad truth is that many pet stores will separate out the "prettiest" rats and sell them as pets and the rest as snake food.   Regardless, no care at all has been done for selective breeding for good health and temperment, and none of them have been handled from birth for sociability, as good breeders will do.

Now your two boys HAVE been handled from birth, so they are sociable for the most part and don't fear humans.  However, the agression in your boy is now showing because likely there is high male hormone levels in his genes.

I'm happy to hear that he's not aggressive to his brother and hopefully that won't change.  Sounds to me as if he is the alpha rat of the two, the dominant one, and his brother is submissive to him, and that they have a good relationship with these roles established.

Still, with neutering, the biting should stop and he should become more laid back, and will end up to be a great pet for you again.  Hopefully, his health will hold out and he will be with you for two or more years, but you should have a good vet selected just in case you need to make emergency visits.

If there's anything else I can help with, please let me know.

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Irene Murphy


I can answer a variety of questions regarding adoption and care of pet rats throughout their lifetimes, including questions about their health and well being, temperment, diet, bedding, cages, toys, etc. I will try to answer your questions to the best of my ability in a timely manner and I have an abundant amount of resources to help me to help you with your pet rats. I love rattie pictures, so include pics with your question if you can. You may ask me medical questions, but please be advised that I am not a vet. I may use my resources to answer some medical questions, however, I will need to refer you to your local vet with medical questions that I feel I am not qualified to address.


I have been a huge rat enthusiast for many years. Since becoming a rat owner, I have educated myself in all areas of pet rats from every resource I could find including the internet, books, conversations with local exotic vets, as well as several local rat breeders.

I have a college degree but not in the area of animals. I have obtained my extensive experience and knowlege of pet rats all on my own because in my eyes, pet rats are the most interesting and fascinating creatures you can ever imagine to have as pets. I also am saddened by how mislabeled and misunderstood these amazing and extremely smart animals are by the majority, and my mission in life has become to educate and change as many people's perceptions of rats as I possibly can.

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