A 2009 PNAS study by Ouattara, Lemasson and Zuberbühler showed that adult male Campbell's monkeys (Cercopithecus campbelli) in Taï National Park, Ivory Coast, combine six loud call types into nine context-specific sequences governed by concatenation principles (e.g., prepended boom pairs signal a nonpredatory context and cancel predation meaning) — described as the most complex 'proto-syntax' then known in animal communication. The authors explicitly flagged as unresolved what this production-side description leaves open: whether receivers extract the proposed sequence-level meanings, and whether males produce the sequences in order to inform others or as byproducts of other motivations. The surviving open problem is the establishment of the system's communicative status: receiver-side validation of sequence-level semantics and the intentional status of sequence production.

Background

Campbell's monkeys (Cercopithecus campbelli) are forest guenons (African monkeys of the Cercopithecus radiation that typically live in one-male groups) of the West African rainforest, where they forage in mixed-species associations with other guenons such as Diana monkeys (Cercopithecus diana). In these associations the single adult male of each Campbell's group is the only producer of loud, long-distance calls. A playback experiment — the standard field method in animal communication — broadcasts recorded calls from hidden speakers and scores listeners' behavioural or vocal responses, allowing inference about what receivers extract from signals.

A long-term field study in Taï National Park, Ivory Coast (Ouattara, Lemasson and Zuberbühler, 2009) showed that male Campbell's monkeys possess six acoustically distinct loud call types — boom (B), krak (K), krak-oo, hok (H), hok-oo, and wak-oo — where krak-oo and hok-oo are the stems krak and hok trailed by an acoustically invariable '-oo' suffix, an affixation that itself alters call meaning (krak is given to leopards, krak-oo to unspecific danger; Ouattara et al., PLoS ONE 2009). Crucially, the males combine these calls into sequences rather than emitting them at random: the study documented 224 calling events organized into nine distinct sequence types, each tied to a specific context — for example, booms alone (male separated from the group, triggering group progressions), booms plus a krak-oo series (falling trees), booms plus krak-oo and hok-oo (neighboring groups), krak plus krak-oo (leopard), and wak-oo, krak-oo plus hok (crowned eagle). Sequence content appeared to be built from call meanings by a small set of concatenation principles: adding a call to a meaningful sequence can change its meaning (boom pairs prepended to alarm series indicate a nonpredatory context — the 'boom cancels predation meaning' principle), adding meaningful calls refines meaning, and calls follow nonrandom ordering rules (booms always first; hok and krak early when threat is high). The authors described this as the most complex example of 'proto-syntax' in animal communication known to date — combinatorial organization of vocalizations that may be an evolutionary precursor of the syntactic organization of human language.

Receiver-side evidence at the time came only from heterospecific eavesdroppers, not Campbell's monkeys themselves: playbacks showed that Campbell's hok/krak series elicit predator-appropriate alarm calls in Diana monkeys, and that prepending boom pairs abolishes those responses, consistent with the boom-cancel principle (Zuberbühler, 2002). Compositional semantics — the property that the meaning of a complex expression depends on the meanings of its parts and the way they are put together — has since been playback-validated in only one wild animal, the Japanese tit (Parus minor), whose receivers extract a compound meaning from 'ABC-D' note combinations and reject artificially reversed orderings (Suzuki et al., 2016, 2017). Formal-semantic analyses of the Campbell's system ('animal linguistics') treat concatenation as conjunction over lexical call meanings, with competition-based strengthening, and have documented a dialectal difference between populations: at Taï, krak is effectively leopard-specific, whereas on Tiwai Island, Sierra Leone — where leopards are absent — krak functions as a general alarm call (Schlenker et al., 2014; Kuhn et al., 2018; Schlenker et al., 2016).

The 2009 authors were explicit that their findings were production-side descriptions: whether receivers actually extract the proposed sequence-level meanings, and whether males produce the sequences in order to inform others — rather than as byproducts of other motivations, such as advertising an uninjured state after a falling tree or providing an acoustic focal point for a scattered group — could not be decided with their data and required systematic playback and motivation experiments.

Problem Statement

For wild Campbell's monkeys, establish the communicative status of the combinatorial call system, in two linked parts. (1) Receiver-side validation: whether the Campbell's monkeys themselves extract the proposed context-specific meanings from natural sequences and from artificially recombined or novel combinations. This includes direct tests of the 'boom cancels predation meaning' principle and of the proposed concatenation rules by which adding, reordering, or recombining calls changes or refines sequence meaning (e.g., boom pairs prepended to alarm sequences versus bare alarm sequences; recombinations of two sequence types into a third). (2) Producer motivation: whether males produce these sequences with the intent to inform others about external events, or whether sequence production is a byproduct of other motivations — such as advertising an uninjured state after falling trees or providing an acoustic focal point for a scattered group — thereby separating intentionally informative production from self-directed or social-display functions.

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