Scientists have discovered that wild orangutans use vocalizations with a layered complexity, a communication method previously considered unique to humans. This finding suggests an older evolutionary origin for this type of communication. The research highlights parallels between orangutan and human communication methods.
The similarity is illustrated by the concept of 'recursion,' exemplified by the phrase 'This is the dog that chased the cat that killed the rat that ate the cheese.' This sentence demonstrates layered complexity through repeated verb-noun phrases like 'chased the cat' and 'ate the cheese'. Recursion involves embedding language elements to form comprehensible thoughts or phrases, allowing for infinite messages with increasing complexity.
Analysis of alarm calls from female Sumatran orangutans revealed a rhythmic structure with self-embedding across three levels. Individual sounds combine into small combinations (first layer), which group into larger bouts (second layer), and these bouts form even larger series (third layer), each with a regular rhythm. This discovery challenges the notion that recursion is exclusively human.
The orangutans also adjusted the rhythm of their alarm calls based on the type of predator encountered. Faster, more urgent calls signaled real threats like tigers, while slower, less regular calls indicated less credible dangers. This structured vocal recursion conveys meaningful information about the external environment.