Are my two feminine rats playing or warfare?

I've had these two pet rats for roughly a month now. They're both womanly, maybe six months antediluvian? (Got them at PetCo in the "Large Rats" subsection.) They seem to gain along pretty well, similar to they groom each other and they sleep together curled up surrounded by the same nest, but a couple times I've see one of them flip the other over on its back and nip at it. It's other the same one on top, I assume. At first I thought they were playing, but the one on its hindmost started squeaking really loudly, and then go very still for nearly thirty seconds. Is this typical? Should I separate them?
Answers: This is very mundane for rats.

There is no reason to separate them.

Hamsters are solitary critters and don't play ably with others.

Rats are massively social and need other rats to be really blissful.

One girl is the alpha and wants to construct sure the other girl remembers it.
She wont hurt her friend, but will flip her and make her squeak. After she squeaks the alpha may a moment ago hold her down or may "power groom" her.

I have 8 rats and the alphas appear to need to save doing that. They live very vivaciously together about 99% of the time.

With my girls, Liilu and Fifi, when Fifi make Liilu squeak I take Fifi for a time out, and she may next go days in need flipping her buddy. Most of the time they are curled up together sleeping.
The same thing happen to my hamsters, I got a cistern seperator. Just to be safe. Things might win nasty.

Good Luck!
It's commonplace. It's just the girls determining 'alpha'. No foundation to separate them if they're not causing impair to each other. My 2 girls still do this, and they've lived together for almost 2 years very soon. Most of the time, it's the same one that's pinning the other one down.

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