Imagine that, instead of macaques, it's members of an isolated Amazonian tribe who have no experience with technology. Now who owns the "selfie" and why?
Without coming to a conclusion on the original subject, only humans are considered people in law as far as I know (in most jurisdictions, right?). So these other cases aren't strictly equivalent. An animal cannot own property, intellectual or otherwise. It cannot sign a contract, nor can it commit a crime. Only humans can.
The question of whether an animal can own property is irrelevant. Copyright is about what the artist does, not what others do. Is there any difference in the actions of photographer in the two hypothetical cases? Answer: no, the photographer's actions are the same. Therefore, if the photographer's actions would not earn him copyright in the case of isolated tribespeople, the same is true for the macaques.
There is a faulty (I believe) assumption that every photo is entitled to copyright, and the only question is who gets it. Several here have implied that copyright should be assigned to the human who has the most to do with the photo (by some vague definition). This, I believe, is false: there is a minimum bar of action that's needed for a photographer to earn copyright over a photo. To my mind that minimum bar is framing the shot. Slater did not frame these shots, so he did not earn copyright. It does not matter who or what might be the subject of the shot, or who else is involved in the shot. If he didn't do that simple act then he doesn't get copyright. The question of whether some other person or entity might be entitled to copyright is entirely separate. It's entirely possible that there is no copyright on these photos at all.
It will be interesting to see if/how this is resolved by an Indonesian court.