there is no such thing as too big an age range with rats.. they are used to living in large colonies with ages from newborn to 4 y/o! Older rats (girls usually) of over a year will generally see babies as something to mother, the alpha rat especially will do this.

as yours are quite close in age the older one is merely saying i'm boss cos i'm older... the fighting you see is likely to be little more than making sure the younger rat knows it.. once the older one is happy that younger has taken this in they will be friends. I wouldn't separate them when they're fighting unless the older one doesn't stop at all.. they need to sort out who's boss and a few squabbles will do this.. it might happen for a few mins at the beginning each time you play with them but it should eventually stop.

If the older rat doesn't stop after a few mins or doesn't get interested in anythign else you give her, treats etc.. then pick her up when she's fighting, stick her on her back say NO loudly and let her struggle out from under your hand... keep doing this if she continually goes to fight with younger rat. If she doesn't comply, stick her in her cage and play with younger rat where she can see you... just like a naughty child.. show her that her behaviour is unacceptable and she won't get to play if she doesn't stop messing about.

when she's being good an ignoring younger give her lots of fuss and attention and treats.

Rats have the intelligence of a 2y/o so don't give up yet. You do get rats that just don't get on either with one rat in particular or any rat, but it's quite rare.. in all the time i've had rats I've had around 10/15 rats) i've only had 1 girl who hasn't gotten on with babies (she was always abit evil) and one boy who we bought knowing this was the problem.

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This is not a game of 'who the fek are you'.

Not a single gram of fek shall be given today.