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Pet Rats/Aggressive rat killed new cage mate


QUESTION: Hello. I am having difficulties with one of my rats - my girl Lily, who is not quite a year old, has been getting increasingly aggressive over past months. I had her in a cage with my older girl Paradox, who is about three years old. Lily came from PetCo with two other babies after Paradox's first friend passed. Both of the other two babies died of a respiratory infection shortly after I brought them home, but Lily recovered on the antibiotics. She and Paradox seemed to get along well enough - they squabble often, but never anything serious.

The problem now is how aggressive she's becoming. I know that Paradox will pass one sooner or later, so I wanted to get a few more babies to introduce who will keep Lily company afterward. I brought home three girls, each just a few months old, this time from a friend who I trust rather than a pet store. I had another friend helping me with introductions and all seemed to be going well when Lily suddenly lunged for my friend's hand and clamped down hard, drawing blood and refusing to let go. We got her off, but she immediately turned on one of the new babies before I could remove her from the group.

Lily had that little girl by the throat and it killed her very quickly; I could not get them apart in time. For now the babies and Paradox are living in the same cage under supervision, as they are getting along splendidly. Lily, however, has been separated.

My mother has suggested that I put Lily to sleep, but I don't really want to do that. At the same time, I don't believe it is good for her to be living alone, and besides the big main cage I only have a small one where the girls go if I can't let them run around while cleaning the big one. I am scared to try and introduce Lily to the new babies again, though, because I have no idea how she'll react. I don't know if she'll try to kill another, or if that was a one-time thing.

Can you please give me some advice on what is going one with my Lily, and what exactly I can or should do?

Thank you,

ANSWER: Hi Kerry,
I'm so sorry for your tragedy. Would it be possible to have Lily spayed? That will reduce her aggression. Otherwise, at this point, seems like the thing to do is just let her live with Paradox, since they are getting along okay, and only attempt introducing new rats after Paradox passes. It is normal for Lily to be aggressive toward strange rats, although she is much more aggressive than normal. She might be more accepting of the new rats after Paradox passes, Lily is alone, and she is more familiar with the scent of the new girls.

Where were the rats when you were doing introductions and Lily killed the baby?

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

QUESTION: I don't think that spaying her would be possible, as the only vet in town doesn't treat much besides dogs, cats, and livestock. Taking her to the vet in a bigger town the way we did for the antibiotics would be more expensive than I can do at the moment.
I had the girls in my bathtub, using it as a neutral location where none of them would feel ownership of the space right away. It's also the easiest place to clean.


I understand about the spaying. Where do you live?  Who would have thought she would kill another rat in the bathtub. You did good, she is obviously just very aggressive. If she won't accept any other rats in the future, she will just have to live alone.

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Debbie Ducommun


I can answer any questions about pet rats, but you will probably be able find answers to simple questions more quickly on my website at If you have a life-threatening emergency you can try calling me at 530-899-0605. I am not usually on the computer on the weekend.


I have been "The Rat Lady" since 1985 and am recognized as one of the world's experts on pet rats. I have 3 published books and already answer lots of questions about rats daily.

President of Rat Assistance & Teaching Society

I am a monthly columnist for Pet Business magazine, and my writing has appeared in other magazines. I have 3 published books.

BA in Animal Behavior

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