Researchers from the University of Warwick have discovered that wild orangutans use vocalizations with a layered structure resembling recursion—previously believed to be unique to human language. This finding suggests that the evolutionary roots of complex communication may be far older than previously assumed.
Recursion involves embedding phrases within phrases, allowing a limited set of elements to convey an infinite number of meanings. A classic example in human language is the sentence: "This is the dog that chased the cat that killed the rat that ate the cheese," where nested phrases build complexity.
Long considered a defining trait of human communication, recursion allows for the expression of nuanced and abstract ideas. However, new research published in Annals of the New York Academy of Sciences challenges this assumption by demonstrating recursive patterns in orangutan alarm calls.
Dr. Chiara De Gregorio, research fellow at the University of Warwick, along with colleagues Adriano Lameira and Marco Gamba from the University of Torino, analyzed vocal data from female Sumatran orangutans. They found that the orangutans' alarm calls were organized in three recursive levels: individual sounds combined into short sequences, those sequences grouped into larger bouts, and the bouts further arranged into extended vocal series—each with a consistent rhythmic structure.
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This multi-tiered structure resembles musical compositions, where rhythms are nested within each other in a repeating and organized way. Remarkably, the orangutans adjusted these rhythms depending on the type of threat they faced. When encountering a real predator like a tiger, their alarm calls were faster and more urgent. When faced with less credible threats, such as a colorful cloth, the calls were slower and less structured.
Such context-dependent modulation indicates that these vocalizations are not random, but instead represent a deliberate use of structure to communicate specific types of danger.
"Our results show that recursion—the foundation of human language structure—was already part of our evolutionary lineage," said Dr. De Gregorio. "Orangutans provide us with an invaluable window into how the basic architecture of language may have emerged millions of years ago."
This is the first empirical evidence supporting the idea that recursive communication did not appear suddenly in humans but likely evolved gradually in earlier primate ancestors.
