Orangutan calls have a complex structure that was thought to be unique to human language. Short sequences are nested inside longer sequences, much like the way we assemble long sentences from shorter phrases.
There is no evidence that any complex meanings are encoded in the orangutans鈥� intricate calls. But the fact that they can do it at all means the calls may be a distant precursor to language.
at the University of Warwick in Coventry, UK, and his colleagues studied wild Bornean orangutans (Pongo pygmaeus) in Indonesian Borneo. They recorded long calls, which are made by larger, more sexually mature males. 鈥淭hey are advertisement calls,鈥� says Lameira, functioning to attract females and deter competitor males. 鈥淭hey take the form of 鈥榦oo ooo ooo鈥�.鈥�
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Lameira wanted to understand the rhythm of the calls. But this proved tricky. 鈥淲e were having difficulties,鈥� he says, because the tempo varied dramatically during each long call: fast bursts of short 鈥渙oo鈥� sounds were mixed with longer, slower 鈥渙oo鈥� sounds, he says.
Closer analysis revealed that the middle of each long call had a regular tempo of long 鈥渙oo鈥� sounds, but the beginning and end had bursts of shorter 鈥渙oo鈥� sounds, with a range of tempos and acoustic features.
Crucially, the shorter sequences were nested inside longer sequences. This resembles a feature of human language called recursion. We can create complex sentences, for example, by nesting an extra phrase in the middle of a sentence, as with: 鈥淭he dog, which chased the cat, was barking鈥�. The orangutans鈥� long calls had a similar recursive structure.
Previously, researchers have failed to find evidence of recursion in the calls of apes, monkeys聽or other animals. The closest anyone has come is 鈥� but this isn鈥檛 the same as observing wild animals actually making recursive calls. Recursion seemed unique to humans.
Lameira says previous searches for recursive calls were unsuccessful because they asked too much of their animal subjects. Humans use recursion to create new and complex meanings, and researchers had looked for animals that do the same. But they seem not to.
His team鈥檚 analysis didn鈥檛 look at the meaning of the calls at all. 鈥淚t was a pure structural analysis,鈥� he says.
鈥淚t鈥檚 quite ingenious,鈥� says at the University of Copenhagen in Denmark. She says the orangutan calls do seem to have a form of recursion, without necessarily any meaning.
Lameira emphasises that there is no evidence the orangutans are encoding complex meanings in their long calls. It may be that the recursive complexity is simply a form of showing off that is similar to birdsong, which can be incredibly complicated without usually meaning anything subtler than 鈥減lease mate with me鈥�.
We should look for similar forms of recursion in other animals鈥� calls, says Briefer. 鈥淚t would be very logical in terms of evolution that we find these precursors. The opposite would be more strange, that we don鈥檛 find anything like that in animals, and it only appeared in humans.鈥�
Some animals can express complex ideas in other ways: and can string a few sounds together in sequence to create meaning. These animals have rules governing the order they make the sounds, like syntax in human language, but they don鈥檛 use recursion to generate meaning.
eLife
