Scientists studying the origins of language have discovered a more intricate and organised vocal communication system among wild chimps, with a dozen different call types integrated into hundreds of different sequences.
The researchers recorded almost 4,800 vocalizations from three groups of chimps in Ivory Coast's Ta National Park, which is one of the last significant remains of old-growth tropical forest in West Africa and home to a diverse range of plants and animals.
Chimpanzees, along with their cousins the bonobos, are the closest surviving genetic relatives to humans. They are clever and highly sociable apes who create and use tools and can learn basic human sign language. Scientists have long known that chimps employ a variety of vocalizations in the wild, but the new study was the first to examine this inter-species communication in detail.
"It's not a language, but it's one of the most complicated forms of communication recorded in a non-human animal," said Cédric Girard-Buttoz, a behavioral ecologist at the CNRS' Institute for Cognitive Science and lead author of the study published this week in Communications Biology.
A grunt, a panted grunt, a hoot, a panted hoot, a bark, a panted bark, a pant, a scream, a panted scream, a whimper, a panted roar, and the non-vocal lip smack and raspberry sounds were among the call kinds. These call types were found in 390 different sequences, according to the researchers.
The order in which the chimps made their sounds appeared to follow rules and structure, albeit the study did not reach any judgments about the cries' possible meanings.
"The key finding is the ability of a primate other than humans to produce several structured vocal sequences and to recombine small sequences with two calls into longer sequences by adding calls to it. It is important because it shows the premise of structured communication which could have been the foundation of the evolution towards syntax in our language," Girard-Buttoz said.
The arranging of words and phrases to create understandable sentences is known as syntax.
"One of the most common sequences is the well-described 'pant hoot' sequence either as 'hoo' plus 'pant hoot' or 'hoo' plus 'pant hoot' plus 'pant scream' or 'pant bark.' But other sequences are also frequent like 'hoo' plus 'pant grunt' or 'grunt' plus 'pant grunt.' In general 'pant grunt' and 'pant hoot' are the most common calls used in these sequences," Girard-Buttoz said.
The researchers want to know if the different sequences express a broader range of meanings in the chimpanzees' complicated social milieu. They have doubts about the meanings of some vocalizations.
"We need to explore in detail the contexts of emission of these vocalizations to see if it shifts between single calls and sequences," Girard-Buttoz said. "Then we need to conduct playback experiments to see if the suspected meaning matches with the behavioral reaction of chimpanzees when they hear the call."
The researchers are unsure if chimp vocal communication is related to the origins of language in the human evolutionary lineage. Humans and chimps share a similar ancestor, yet 7 million years ago, they split into separate lineages.
"Protolanguage was probably somewhere between chimps and humans," Girard-Buttoz added.
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