People will argue fervently about how maternal aggression is understandable to a degree, or entirely unacceptable – but we run into a huge communication issue because there’s no definition of what “maternal aggression” is, and so quite often people are talking at cross purposes.
The last time I got a bite from a doe with babies (not new babies – a couple of weeks old), she raced across the cage and bit me when I was nowhere near her babies – I consider that maternal aggression. She was absolutely fine before her babies, and absolutely fine once removed from them (the temperament of her kittens was, and remained, superb). I think we would all consider that maternal aggression without much debate, and all of us would be happy to consider that undesirable behaviour in a doe.
The edges is where it gets more murky. I’d never even consider putting my hands under a doe with new kittens to yank her kittens out from under her – it just wouldn’t occur to me as a respectful or appropriate way to handle a rat. Is that doe maternally aggressive? She hasn’t shown any aggression (my does generally come off the nest to get their treats, at which point I take the babies out). But if she didn’t immediately come off the nest and instead sat and fussed over her kittens and made it clear through her body language that she wasn’t comfortable leaving her newborns, I wouldn’t be concerned about that – she might just not want to right now, something might have spooked her or made her unhappy, she might just not feel like it. Some people want to be able to rip her off the babies and take them out without any unhappiness from the doe. It seems to be often a cultural thing in that different groups of fanciers consider this to be appropriate/acceptable.
The problem comes when people insist that the second example provides evidence of maternal aggression, that accepting that behaviour shows a breeder to be unethical or somehow subpar, and often insist it proves that the kittens will be bloodthirsty angry aggressive killers – rather than just accepting that different people raise rats in different ways, and all of us are perfectly capable of producing perfectly fine rat kittens. If you tell me that a doe who gives me a warning nip if I reach under her to take out a newborn pinkie is evidence of poor temperament in a line, I’m going to have to ignore what you’re saying because the thousands of kittens I’ve produced from does like this with perfectly sound temperaments proves that your hyperbole is wrong. You’re fine to not want to breed from does like that yourself, of course – but it’s silly to point to the probably hundreds people successfully doing something (for example – 99% of the UK fancy, who would consider someone getting a nip from a new mother as they reached under her to have got what they deserved) and tell them it’s impossible.
Equally, if you feel that behaviour from a doe is not desirable and want to select away from it, that is an entirely reasonable aim to have, but not all breeders will consider that a priority area for selection or even a necessary area for selection, and all of those viewpoints are equally valid to hold.