What's the ape all about? Why spend an entire story detecting the activities of an animal that can't be held accountable for its actions? We're not sure if there's an easy answer to this question, but one idea we had is that the text of "The Murders in the Rue Morgue" seems to be preoccupied with the division between creativity and analysis. However, it bypasses another important psychological split between the rational and emotional minds and the division between the brain and the body. The Ourang-Outang (remember, that's Poe's old fashioned spelling of "orangutan") has a lot of symbolic value in this latter two splits.
We said, in our "Character Analysis" of the narrator, that Dupin and the narrator seem functionally like two halves of one person. But we think this might be true of Dupin and the Ourang-Outang, too. After all, Dupin's superior guessing skills wouldn't be needed if the Ourang-Outang hadn't done something so strange that the police couldn't figure it out. And we wouldn't even know about the Ourang-Outang if Dupin wasn't a super genius. Dupin is the smartest of the smart and the Ourang-Outang is the most emotional of the emotional – he slashes a woman's throat because she won't let him shave her.
In a story that celebrates the height of logic, it makes sense that the killer puts the "beast" in "bestial." After all, Poe is trying to show that Dupin's particular brand of rationality can battle even things that have no rhyme or reason of their own. What provides the greatest challenge in the case is precisely that it is beyond human understanding, beyond motive, and constitutes the ultimate crime of passion. The ape comes to the L'Espanaye home wanting to play, and when his newfound friends don't join in, he kills them in a fit of rage. And then he flees from his sailor master in guilt and fear. But Dupin is so great that his brain can bring even this extreme emotion of the ape into a logical story from Point A: Borneo to Point B: Central Paris to Point C: Madame L'Espanaye's apartment.
This story is less about punishment than it is about controlling what's unpredictable, in the individual and in society. The Ourang-Outang is like that part of Dupin (and of every person) that we struggle with, the completely irrational part that might do something crazy at any moment. This bit isn't exactly a stretch – after all, the Ourang-Outang escapes from a closet, where it's been stuffed to keep it under control. It's not much of a jump to think of this symbolically as a representation of an individual's efforts to keep bodily desires and feelings under wraps.
On a larger level, we also wonder whether there might be something to the fact that "The Murders in the Rue Morgue" is itself a puzzle, and the ape is also a gamer. The murder is triggered because of the ape's desire to play barber, first with itself, then with Madame L'Espanaye. And we're reading "The Murders in the Rue Morgue" to follow our desire to play detective.
So Poe's "tale of ratiocination," this experiment in a short-story-as-puzzle, is like a logic game that should fulfill our human need to play without all the messy emotion that might bring about strangulation. The story itself is working to stimulate our brains but not our hearts – deliberately avoiding all emotion except horror at the antics of an ape.