This is a short story from the 1980s, in Amazing or F&SF I think. It's told in the first-person by a college professor (of English, I think) who finds himself in the body of an ape in the rain forest. I think the transition occurred while the professor was at a faculty meeting, disputing things with a rival; as an ape, he can't speak, and he gradually becomes more and more ape-like in thought, joining with nearby apes in fighting those from another tribe, while still holding onto the idea that he is really the professor he remembers being, and wondering if any of the other apes are also former humans (I think he hopes that his professorial rival is one of the members of the other tribe).
P.S. Throwing feces is one of the initial means of battle between the ape groups, if I recall correctly.