Wild bonobos, our closest living relatives, are able to produce vocalisations in a wide range of emotional states and situations just like human babies, according to a new study which suggests that core features of human language may have deep roots in primate lineage.
Researchers studied wild bonobos and found that they produce a call type, known as the 'peep', across a range of positive, negative and neutral situations, such as during feeding, travel, rest, aggression, alarm, nesting and grooming.
Peeps are high-pitched vocalisations which are short in duration and produced with a closed mouth.
The researchers from University of Birmingham, UK and the University of Neuchatel, Switzerland, looked at the acoustic structure and found that the peep calls did not vary acoustically between neutral and positive contexts - for example, between feeding, travelling and resting.
Similar to human infants, recipients therefore have to make pragmatic inferences about the meaning of this call across contexts.
"When I studied the bonobos in their native setting in Congo, I was struck by how frequent their peeps were, and how many different contexts they produce them in," said lead author Zanna Clay, from the University of Birmingham's School of Psychology.
"It became apparent that because we couldn't always differentiate between peeps, we needed to understand the context to get to the root of their communication," she said.
The common assumption is that primate calls are tightly tied to certain contexts and emotional states, whereas many human vocalisations are freed from this.
However, before language develops, human babies can produce a group of calls - called 'protophones' independently of their emotional state.
These types of infant calls differ from commonly recognised calls such as laughter and crying, and the calls of most animals, which are thought only to be produced in certain contexts.
"We felt that it was premature to conclude that this ability is uniquely human, especially as no one had really looked for it in the great apes," Clay said.
The type of communication the researchers observed in the wild bonobos could represent an important evolutionary transition from 'functionally fixed animal vocalisations' towards human vocalisations, which seems to have appeared some 6 - 10 million years ago with the bonobo, our shared common ancestor with apes.
It appears that many of the core features of human language have deep roots in the primate lineage, researchers said.
The study was published in the journal PeerJ.
