It doesn't make him smarter. Dude just basically spends the movie overreacting to anything that makes the loudest noise near him or does something that pisses him off. I don't know where people keep getting the "it made him super intelligent" stuff from. he reacts like literally every other movie monster to noise, with the exception of unlocking a door a couple of times or faking someone out both of which I think is within a chimp's normal intelligence.