In groundbreaking work from The University of Warwick, researchers have found that wild orangutans vocalise with a layered complexity previously thought to be unique to human communication, suggesting a much older evolutionary origin.
Consider the phrase – 'This is the dog that chased the cat that killed the rat that ate the cheese'. It is a simple sentence comprised of repeated verb noun phrases - 'chased the cat', 'ate the cheese' - and is an example of layered complexity called recursion.
Recursion is the repetition of language elements in an embedded way so that they form a comprehensible thought/phrase. Like Russian nesting dolls, the power of recursion mean we can combine a finite set of elements to deliver an infinite array of messages with increasing complexity.
It is widely believed that nested communication is a unique feature of human language, allowing us greater complexity of thought, but research from The University of Warwick, published today in Annals of The New York Academy of Sciences , tells a different story.
Dr. Chiara De Gregorio, Research Fellow at The University of Warwick, who performed this work alongside Adriano Lameira (also Warwick) and Marco Gamba (University of Torino), said: "When analysing the vocal data of alarm calls from female Sumatran orangutans, we found that the rhythmic structure of orangutans' sounds made were self-embedded across three levels - an impressive third-order recursion. Finding this feature in orangutan communication challenges the idea that recursion is uniquely human."
The three-layered (recursive) structure of the orangutan's calls was as follows:
- Individual sounds made by orangutans occurred in small combinations (first layer)
- These combinations could be grouped into larger bouts (second layer)
- And these bouts could be grouped into even larger series (third layer), all with a regular rhythm at each level
Just like a musical piece with repeating patterns, orangutans nested one rhythm inside another, and then another, creating a sophisticated multi-layered vocal structure, not thought possible by non-human great apes.
This pattern wasn't accidental because orangutans also changed the rhythm of their alarm calls depending on the type of predator they encounter: When they saw a real threat, like a tiger, their calls were faster and more urgent. When they saw something that seemed like a threat but lacked the credibility of a real danger (like a cloth with colourful spots), their calls were slower and less regular.
This ability to adapt vocal rhythms to different dangers shows that orangutans aren't just making noise, they are using structured vocal recursion to carry meaningful information about the outside world.
"This discovery shows that the roots of one of the most distinctive features of human language — recursion - was already present in our evolutionary past," adds lead author Dr. De Gregorio. "Orangutans are helping us understand how the seeds of language structure might have started growing millions of years ago."
This research presents the first empirical support for the idea that these powerful recursive capacities could have been selected for and evolved incrementally in a much earlier ancestor.
