Why are my female mice humping each other?

I recently purchased 2 fancy females and I am finding that one of them is constantly sniffing the other's bottom and has taken to humping her. I've consulted several websites in regards to sexing them and I'm 99% sure they are both female. The one being humped doesn't seem to enjoy this behaviour from the other one; is the humper trying to assert dominance over the other?
Answers:
horny?
yes, it is a dominant thing
In female dogs- this behavior is to establish dominance. although I have two female dogs and the small one humps the bigger one when the bigger one has a treat. It seems to annoy the big dog enough that it lets the little dog have the treat.
Yes, it's a dominance thing. You need to separate them before one of them gets killed (happened to my sister's hamsters - the humping turned to fighting and then one of them killed the other one).
They do that sometimes. But all of the mice I have raised have not.
You might want to check their sexes again.

if they are over 3-4 weeks old squeeze their stomachs a little and the males will have testicles that "pop" out a bit. They are clearly visible.
it could be a dominate thing, or maybe that 1 mouse is in heat. because they will hump each other then. but im not sure how long they stay in heat for. or maybe check again if they are both definatly females. ( are they both the same size got the same shaped head? and do there privates look the same?
Animals hump each other to establish dominance over each other. Even animals with hump their owner to show who is the boss.

I hope I've helped you.=)

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