An article passed on by Ken Ehrensal from the New York Times on Campbell’s monkeys having grammar. The grammar is pretty simple but looks like it might be more complex than Vervet monkeys’ efforts. [Link to article.]
The full length article it is based on can be found here: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0007808 and discusses the flexibility of monkey calls and the different kinds of calls including “hok” “hok-oo” and “boom.” Some audio clips accompany the article.