After taking a look at Orangutan Mothers Discuss the Past, it is worth looking at some classic studies on Nim Chimpsky.
More than 19,000 multisign utterances of an infant chimpanzee (Nim) were analyzed for syntactic and semantic regularities. Lexical regularities were observed in the case of two-sign combinations: particular signs (for example, more) tended to occur in a particular position. These regularities could not be attributed to memorization or to position habits, suggesting that they were structurally constrained. That conclusion, however, was invalidated by videotape analyses, which showed that most of Nim’s utterances were prompted by his teacher’s prior utterance, and that Nim interrupted his teachers to a much larger extent than a child interrupts an adult’s speech. Signed utterances of other apes (as shown on films) revealed similar nonhuman patterns of discourse.
Apes really are not humans.