Why is my female gerbil doing this?

i just got 2 female gerbils named shadow (black) and cloud (white) and cloud keeps mounting shadow to show dominance. will she constantly do this or just in the beginning? and is she doing this because they were in a group of females and there was probably a more dominant one in that group and now that i took thoughs two they are trying to figure our who is more dominant out of them? thanks :)