Diana Monkey Alarm Calls — Cross-Species Reception
Female Diana monkeys (*Cercopithecus diana*) produce predator-specific alarm calls in the high canopy forests of West Africa. What makes this entry distinctive within the referential signaling cluster is not the primate-to-primate communication but the documented cross-species comprehension: yellow-casqued hornbills and other forest birds can discriminate between Diana monkey alarm call types and respond appropriately.
What the Signal Is
Female Diana monkeys produce at least three functional call categories:
- Alarm calls — predator-specific, distinct structure for different threat classes (aerial vs. terrestrial predator)
- Alert calls — lower-urgency detection signals
- Greeting/contact calls — social maintenance
Female vocal repertoire is larger and more variable than male repertoire for alarm contexts — an unusual sex difference for referential signals, which are more commonly attributed to males in primate studies.
What It Encodes
Predator class identity. Distinct calls for crowned hawk-eagle (aerial) versus African leopard (terrestrial) have been documented. The receiver behavior is predator-appropriate: eagle alarm → scan upward, crouch in foliage; leopard alarm → scan ground level, move up in canopy.
Diana monkeys also direct calls at distant competitors and neighboring groups, not just local conspecifics — the signal serves both within-group coordination and between-group threat signaling.
Cross-Species Reception: The Hornbill Finding
Rainey, Zuberbühler & Slater (2004, *Proceedings of the Royal Society of London*) demonstrated that yellow-casqued hornbills can discriminate between Diana monkey eagle alarm calls and leopard alarm calls. Hornbills exposed to playback of eagle alarms showed flight/cover responses appropriate to aerial predators; leopard alarms triggered different responses.
This is one of the cleanest demonstrations that referential signals can bridge radically different Umwelten. The hornbill has no evolutionary history with the Diana monkey — they share only a common predator threat landscape. The hornbill has evolved to read the monkey's signal.
A parallel mutualistic system documented in India: axis deer and Hanuman langurs form a cross-species early warning network, each species alerting the other to different predators using their own calls. The Diana/hornbill system appears to be an asymmetric version — the monkey produces, the hornbill receives — though Diana monkeys may also respond to hornbill alarm calls.
Honest or Deceptive
Honest. The predator-specific call structure is costly to produce (energetically and by attracting predator attention to the caller), which maintains honesty. There is no documented deceptive use in Diana monkeys, though the related fork-tailed-drongo-alarm-calls (to be written) shows how a third species can exploit primate alarm call systems kleptoparasitically.
Key Researcher / Discovery Story
Klaus Zuberbühler (then at St Andrews, now Neuchâtel) has been the primary researcher on Diana monkey communication, working in Taï Forest, Côte d'Ivoire. His program expanded from Diana monkeys to a multi-species approach to Taï Forest communication, showing that multiple sympatric species (Campbell's monkeys, chimpanzees, hornbills, Diana monkeys) form overlapping signal networks.
The hornbill finding was notable for extending the referential signaling question outside of primates in a direct experimental design — not just "does this species have alarm calls" but "can a non-primate receiver extract referential content from another species' alarm calls."
Open Questions
- What is the mechanism by which hornbills acquire the ability to read Diana monkey calls? Is it innate template-matching to signal acoustic features, or learned association?
- Do Diana monkeys respond to hornbill alarm calls, completing a mutualistic loop?
- The male/female vocal asymmetry is unexplained — why would females carry the more complex alarm repertoire?
- How do Diana monkey calls interact with Campbell's monkey calls in the overlapping territories of Taï Forest?