What did The Planet of the Apes' society do with the bonobos?

Although Pierre Boulle's original novel and Charlton Heston's movie (plus its sequels) differ a lot, they coincide in one thing:

Ape society is composed of all apes, with the exclusion of humans: chimpanzees, gorillas and orangoutans are together as equals.

There's a problem with this... what about bonobos? This omission's real reason is rather prosaic: back when both the book and the movie's script were written, it was believed bonobos were chimpanzees, something that we know today not to be the case. If we set aside this inconvenient detail, this omission's implications are rather worrying, specially in the movie (the book still has the excuse of taking place in a different planet). There are two possibilities:

1) Bonobos are extinct. If so, how and why? Who did it?

The movies show an ape society rather small both in population and territory occupied (a great deal is made about expanding to the "Forbidden Zone"). What do these apes know about Central Africa? Is there any ape presence over there?

2) Bonobos are alive. This possibility is even more worrying: if they're alive, where are they? Knowing their peaceful attitude and their main mechanism of social interaction, everything points to bonobos being sex slaves, locked up in other apes' homes. Disturbing, isn't it?