sudden aggression in female rat

Question
QUESTION: Hi,
My uncle lives with 3 female rats. Just today, one of them has suddenly become quite aggressive towards her cagemates (one in particular). They often play fight, but this time she has her hair raised and moves sideways towards them. She's defending the travel cage she likes, not letting the others in there, but will fight with them inside the cage too. She just recently threw one of her cagemates off the top ledge, which was quite worrying.
We've been concerned that she's got pregnant, but can't really imagine it having happened. I live with two male rats who occassionally are in the same house as the females (not the same cage of course). About 3 weeks ago one of my relatives was handling one of the males, and got distracted. He slipped away and was found just inside the door to the girls' cage which had been accidentally left open, however they girls were still in the nesting box. I checked them over right away and couldn't see any indication they had mated. He could have only been in there for a moment.
Do you have any thoughts as to the cause of the aggression, or possible temporary solutions? He's already provided the travel cage inside the main cage so the aggressive female can be separate, and has removed the top ledge so none of them can be knocked off that height again.

ANSWER: Hi Skye,
Some female rats become aggressive as they become more mature. How old is she?  She will probably need to be spayed to reduce the aggression.
Deb

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

QUESTION: Thanks for the response.

She's about 6 months we think, and she has continued being aggressive. She isn't aggressive all the time, though; she has times where she will sleep comfortably with her cagemates.

A bit of history is that she (Seven) was adopted, on her own, from the adoption centre at the pet store (she was the only one there at the time) with the intention of getting her some friends. The other two (Olaf and Kristoff) came from a family who'd had an accidental litter, and so those ones had been socialised from birth. Seven has always been a fair bit bigger than them, but much more nervous of people and things in general. They got on quickly when introduced, and Seven became less nervous.

Olaf was always the dominant one, but now it's seeming more and more like Seven is. Kristoff is being picked on by the both of them, though,  and Olaf isn't recieving as much trouble from Seven. Kristoff is becoming more nervous quickly, and runs away from Olaf even when it doesn't seem like she wants trouble.

It'd be preferable if we could avoid spaying, as that's such a risk with such small animals. Do you suppose this could be a change of power, and something that will become less of a problem in the future, or does it sound like spaying will be the only option for us?

Answer
Maybe you can train her to be less aggressive if you give her a timeout in a small bare cage every time she is. The problem is that you won't always be there.

Spaying is not high risk as long as the surgeon knows what she is doing. I'm not a vet and yet I do spays all the time without any problem.  See the article about Surgery on my website at www.ratfanclub.org for what to discuss with your vet.
Deb