Scientists have found that wild orangutans vocalize with a layered complexity. This is a form of communication previously thought to be unique to humans, suggesting a much older evolutionary origin. The research draws parallels between the way orangutans communicate and humans.
This is through a layered complexity previously thought to be unique to human communication, according to The University of Warwick.
The researchers illustrate the similarity with phrase — ‘This is the dog that chased the cat that killed the rat that ate the cheese’. This is a simple sentence comprised of repeated verb noun phrases — ‘chased the cat’, ‘ate the cheese’. The phrase is an example of layered complexity called ‘recursion’.
There are different definitions of recursion, depending on the context. Often recursion reflects the process of solving large problems by breaking them down into smaller, simpler problems that have identical forms.
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It can also mean, from a linguistics perspective, the repetition of language elements in an embedded way so that they form a comprehensible thought/phrase. This means we can combine a finite set of elements to deliver an infinite array of messages with increasing complexity.
Until now, it was believed that such nested communication was a unique feature of human language, allowing us greater complexity of thought. The new research suggests otherwise.
When the researchers analysed the vocal data of alarm calls from female Sumatran orangutans, they found that the rhythmic structure of orangutans’ sounds made were self-embedded across three levels — third-order recursion:
• 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
This has caused the researchers to indicate that finding this feature in orangutan communication challenges the idea that recursion is uniquely human.
Does this mean humans are more closely related to orangutans than first thought?
Chimpanzees share a very high percentage of their DNA with humans, making them our closest living relatives. However, the actual relationship between humans and primates might be more complicated than this. Genetics research indicates that although orangutans and humans are more distantly related, some regions of our genomes are more alike than those of our closest living relative, the chimpanzee.
Some researchers base their conclusion on a close physical resemblance between orangutans and humans. They argue these physical similarities have been overshadowed by genetic evidence linking us to chimpanzees.
Could the vocalisations be erroneous?
The vocal pattern observed by the researchers was not accidental since the 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. In other words, they were using structured vocal recursion to carry meaningful information about the outside world.
The research appears in the journal Annals of the New York Academy of Sciences, titled “Third‐order self‐embedded vocal motifs in wild orangutans, and the selective evolution of recursion.”
