The study, published in the Annals of the New York Academy of Sciences, focused on the alarm calls of Sumatran orangutans. It revealed that these primates can arrange their vocalizations into self-embedded structures spanning three layers, a phenomenon known as third-order recursion.
Lead researcher Dr. Chiara De Gregorio, along with colleagues Adriano Lameira (also from Warwick) and Marco Gamba (University of Torino), noted that individual sounds made by orangutans form small combinations, which in turn can be grouped into larger bouts, and then further combined into even larger series. Each layer maintains a distinct rhythmic structure, much like the nested phrases in human language.
This multi-tiered structure was not random. The study found that orangutans adjusted the rhythm of their calls based on the perceived threat level. For instance, rapid, urgent calls were used when encountering a real predator, such as a tiger, while slower, less regular rhythms were observed in response to more ambiguous threats.
"This discovery shows that the roots of one of the most distinctive features of human language - recursion - were already present in our evolutionary past," said Dr. De Gregorio. "Orangutans are helping us understand how the seeds of language structure might have started growing millions of years ago."
This research offers the first empirical evidence that these complex recursive capacities could have evolved incrementally in a common ancestor, potentially reshaping our understanding of the origins of human language.
Research Report:Third-order self-embedded vocal motifs in wild orangutans, and the selective evolution of recursion
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