On the -oo Suffix of Campbell's Monkeys

1 Introduction

1.1 Primate Morphology?

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 al. 2014) that there is redundancy between the apparent semantic contribution of -oo and the semantic contribution of a variety of other signal manipulations (e.g., calling rate) that are easiest to explain via noncom-positional mechanisms. These facts warrant particular caution when evaluating the pattern as a possible counterexample to generalizations about human language.

Thus, in this squib, we examine the compositional hypothesis further. As counterpoint, we consider a class of more conservative hypotheses in which -oo does not itself bear meaning, but instead arises as the side effect of other articulatory processes that noncompositionally affect call meaning. Key to such hypotheses is the premise that -oo is articulatorily parasitic on another phonetic process. A major contribution of this squib is thus phonetic: considering the acoustic properties of -oo, we conclude that complex calls (krakoo and hokoo) are produced with two pulses of a single breath-group. Critically, the production of these complex calls requires an additional articulatory gesture and thus an increase in articulatory effort. An increase in articulatory effort would not be expected on an analysis in which -oo arises as a phonetic side effect; we accordingly reject these alternative hypotheses, thus strengthening the robustness of the combinatorial analysis.

1.2 Merge as the Putative Defining Feature of Human Language

Bolhuis et al. (2014), following Chomsky (2000), defend the strong hypothesis that the distinguishing feature of human language is the presence of hierarchical syntactic structure. In their words, "[H]uman language syntax can be characterized via a single operation that takes exactly two (syntactic) elements a and b and puts them together to form the set {a, b}." This operation, called Merge in the Minimalist tradition (Chomsky 2000), allows two elements that are themselves syntactic units to be combined into a complex unit that can serve as the input to another combinatory operation. In human language, this second operation might be a further application of Merge, thus recursively generating structures of arbitrary length.

Of course, the presence of Merge does not guarantee the existence of arbitrarily long sequences; note, for example, that the phrase structure grammar with the terminals {D, N, V} and the rules {S → NP VP, NP → D N, VP → V NP} produces sentences with hierarchical structure, but only generates five-word strings. Relatedly, Rizzi (2016) observes that recursive applications of Merge depend on the presence of a "temporary workspace," short-term memory storage for nonlexical inputs to Merge. Without this workspace, a system can produce binary strings of lexical elements, but cannot store these units for further applications of Merge. For Bolhuis et al. (2014), all nonhuman animal communication systems disallow hierarchy of any depth. As indicated above, the alarm calls of Campbell's monkeys pose a potential counterexample; this is thus the question that we address here.¹

2 Male Campbell's Monkey Alarm Calls

2.1 Complex Calls

Male Campbell's monkeys (Cercopithecus campbelli) produce at least three distinguishable alarm call stems (i.e., calls not followed by -oo), called krak, hok, and boom, classifiable both by ear and automatically (Ouattara, Lemasson, and Zuberbühler 2009a,b, Keenan, Lemasson, and Zuberbühler 2013).² The boom call...