Campbell's Mona Monkey — Predator-Specific Alarm Calls and Call Combinations

Campbell's mona monkeys (*Cercopithecus campbelli*) of West Africa produce a small set of discrete alarm calls that encode predator type and urgency — and, crucially, combine those calls using a suffix morpheme that modifies meaning. This is one of the most compelling cases for proto-syntax in animal communication: a non-human species demonstrably combining call units to produce meanings not present in the individual units.

What the Signal Is

Males produce two primary alarm call types:

• "Krak" — a loud, sharp call given to leopards (terrestrial predators)
• "Hok" — a softer, rising call given to crowned eagles (aerial predators)

These calls elicit predator-appropriate responses in conspecifics: "krak" triggers ground-level vigilance and movement toward cover; "hok" triggers upward scanning and movement into lower vegetation.

But males also produce modified versions:

• "Krak-oo" — "krak" + "-oo" suffix: given to a wider range of disturbances, including falling branches and non-predator disturbances; receivers treat it as a general disturbance alert, not a leopard-specific call
• "Hok-oo" — "hok" + "-oo" suffix: given to general aerial disturbances, including non-eagle birds; receivers treat it as a general aerial alert

The "-oo" suffix systematically broadens the referent of the base call. This is the key finding: the same suffix applied to different base calls produces predictably modified meanings in both cases — a morphological rule, not arbitrary variation.

What They Encode

The base calls encode specific predator identity. The suffixed calls encode a broader threat category. Together the system can express:

That final combination — hok calls followed by krak calls — is produced specifically when males hear Diana monkey alarm calls (from a neighboring species), and it appears to function as a "recruit-to-investigate" signal rather than a pure alarm. It is not reducible to either base call's meaning.

Honest or Deceptive

Honest. Unlike the fork-tailed drongo that exploits alarm call comprehension for kleptoparasitism, Campbell's monkey calls appear to accurately reflect the nature of the perceived threat. The broadened "-oo" calls are not false alarms — they're used when genuine but less specific threats occur.

Acquisition

Innate structural framework, with evidence of social refinement. Young males produce the call types from early in development, suggesting the basic phonetic forms are innately specified. However, the appropriate *contexts* for call use — particularly the nuanced distinction between base and suffixed forms — appear to require social learning. Young males show looser context-specificity that tightens with age and social experience.

Key Researcher / Discovery Story

Karim Ouattara, Alban Lemasson, and Klaus Zuberbühler's 2009 *PNAS* paper "Campbell's monkeys use affixation to alter call meaning" reported the systematic "-oo" suffix effect, generating substantial debate in animal communication and linguistics.

The finding was significant because it suggested rule-governed combination — not just a fixed set of discrete calls, but productive modification of existing calls by an appended morpheme. Linguists were cautious: the "suffix" interpretation requires that the "-oo" be genuinely compositional (the meaning of the combination is derived from the meanings of the parts) rather than a separate call that happens to sound similar. Subsequent playback studies by Zuberbühler and colleagues supported the compositional interpretation, but the debate over whether this constitutes "syntax" continues.

The cross-species reception is also notable: Diana monkeys (*C. diana*) and other sympatric species in the Taï Forest eavesdrop on Campbell's monkey alarms and respond appropriately — extending the signal's reach beyond the producing species' own social group.

Open Questions

• Is the "-oo" modification genuinely compositional? Or are suffixed forms learned as holistic units that happen to be phonetically similar to base calls? The former would imply a morphological rule; the latter a larger but still fixed repertoire
• Female call structure: Most research focuses on male calls; female vocal behavior in this species is comparatively understudied
• Ontogeny of call combinations: How do juveniles learn when to produce Hok+Krak sequences vs. individual calls? Is there a critical period?
• Geographic variation: Do Campbell's monkey populations in different parts of West Africa show dialectal variation in call forms or usage contexts?

See Also

• vervet-monkey-alarm-calls — the founding referential signalling study; simpler system (three calls, no combinations)
• diana-monkey-alarm-calls — sympatric species that receives and responds to Campbell's monkey calls
• black-capped-chickadee-dee-count — another alarm system where call modification (dee count) encodes graded information