My 5 month old girls (ND) have been spending a good part of each day mounting each other and smelling each other's rear ends. Prior to two weeks ago, they never displayed this behavior. It is possible, although I'm not positive, that they both went into heat last weekend. I didn't actually see discharge (like I do with my older doe), but they had some dirt & grass stuck on/near the vulva. So if that's the case, all of this mounting seems fairly normal (I think). Also the more dominant of the two is doing the majority of the mounting, so perhaps it's just settling some hierarchy of dominance?