had them about 7 weeks we had some problems at first but everything seems to be great now and they seem to be good mates . but lately for about the last 3 weeks one of them is rumbling and shaking her bum at the other they are both girls and we asked on here if this was natural and people said it was. they are both girls but the same one is always doing it to the other nearly every time she goes past. people told us it was all about mating or dominance but should it happen all the time thanks danny