Last time I promised that I would elaborate on the situation I presented last time, namely, Gertrude and Hilda. To my trained but imperfect human eyes the two look like sisters. As an exercise, let's assume that they are. But if they are, how and why do they appear to differ so greatly in rank?
A few things first. If the two are sisters, they are almost certainly not full sisters, they probably only share one parent. This I shall assume, as males generally do not maintain their rank long enough for a single copulatory pair to make it through two pregnancies. This would also require Gertie and Hilda be born one after the other; there are just too many unlikely aspects to such a setup. Thus we'll assume they shared only one parents, but which parent will prove to be an interesting question.
Sharing a mother between the two sisters is problematic. If this were to be the case, the mother would need to be quite highly ranked, to explain Hilda's own rank. Gertrude's low rank is less of a problem this way - female's can lose a great deal of rank much more easily than they can gain rank. So under this scenario, how does Gertie lose all of her ranks?
Alright, we don't really know enough about Gertrude's rank to say where she's fallen to. Maybe she never really collapsed, maybe she just got super old and couldn't protect herself, or developed some disease which makes her weak. Even though males are the sex whose rank is primarily based on pure strength, and females can rely on the cushion of their matriline, they still need to be able to protect themselves. in truth, Gertie is very seldom bothered by anyone because she stays out of the way and is no longer sexually receptive. In theory she could still hold a high rank, but just doesn't make use of it.
What if they shared the same father? Well, an alpha male is likely to mate with the top rank females, and create progeny of high rank, like Hilda. However, males are never long in the alpha spot. An up and coming male may have joined a troop and impregnated a mid-to-low rank female, and several years later, having risen to alpha status, impregnated a high ranking female who in turn gave birth to Hilda.
On the other hand, a low ranking, but clever male may have regularly mated with low ranked females, but slipped in a few sneaky, extra-pair copulations with a high ranking one, and one such copulation might happened to have yielded a successfully pregnancy. This isn't a terribly likely option, but either scenario would give us Gertrude and Hilda as they are today.
So there you have it, a wealth of different ethologically sound (I swear) explanations for how two old sister baboons ended up in such different places in the autumn of their lives.