Ouattara, Lemasson, and Zuberbühler (2009a,b) make the novel claim that Campbell’s monkey alarm calls demonstrate a simple pattern of linguistic morphology. The authors observe that there are at least two distinct alarm calls (called krak and hok) that are used in two different predatory contexts, and that each may be followed by a low-frequency sound (called -oo) that alters the meaning of both calls in predictable ways, allowing contexts with a reduced level of threat. In light of these facts, -oo is analyzed as a meaning-bearing, combinatorial unit.
However, the claim that a nonhuman communication system has a combinatorial system (however primitive) is rare in the literature (see section 4 for related patterns) and, indeed, is antithetical to certain claims that structural hierarchy is unique to human language (e.g., Bolhuis et al. 2014). Moreover, it has been noted (Schlenker et...