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University of Warwick researchers revealed that orangutans vocalize with a multilayer complexity traditionally assumed to be exclusive to human interaction, suggesting a considerably older evolutionary basis.
Recursion embeds language fragments to produce a thought/phrase. An example of recursion would be “This is the eagle that killed the rabbit that ate my carrots.” Recursion lets us combine a finite collection of pieces to provide an unlimited range of increasingly complicated messages.
The University of Warwick’s Annals of the New York Academy of Sciences study contradicts the belief that layered communication is a unique property of human language that allows us to think more deeply.
The rhythmic pattern of female Sumatran orangutans’ alarm calls was self-embedded over three layers, an astonishing third-order recursion, according to the researchers. This orangutan communication capability challenges the concept that recursion is human-only.
Orangutan calls were three-layered (recursive): Orangutans made little combinations of sounds (first layer); This combination could be organized into larger bouts (second layer); These bouts can be grouped into a third layer with a steady rhythm.
Like musical patterns, orangutans layered one rhythm inside another, generating a complex multi-layered vocal structure that non-human great apes could not achieve.
The vocal sequence wasn’t coincidental because orangutans modify their warning call rhythm depending on the danger. When they saw a tiger, they called in a quicker and more urgent manner. However, the calls were slower and less frequent when they perceived something that appeared to be less threatening.
Orangutans’ ability to alter vocal rhythms to varied dangers reveals that they use organized vocal recursion to communicate with others.
This result suggests that recursion, a key element of human language, evolved early on. Orangutans are helping scientists understand how language structure may have developed millions of years ago.
This first empirical evidence suggests that these powerful recursive abilities were selected for and evolved gradually in an earlier ancestral species.
Sources:
Chiara De Gregorio et al. (2025). Third‐order self‐embedded vocal motifs in wild orangutans, and the selective evolution of recursion, Annals of the New York Academy of Sciences. DOI: 10.1111/nyas.15373
Communication complexity, once thought to be uniquely human, discovered in orangutans. https://phys.org/news/2025-05-communication-complexity-thought-uniquely-human.html
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