Image copyright Zanna Clay/Lui Kotale Bonobo Project

Wild bonobos use a single high-pitched call in a variety of contexts, showing a flexibility in their communication that was thought to be uniquely human.

That is the conclusion of a study by UK and Swiss psychologists.

Bonobos are just as closely related to humans as chimpanzees, but their wild communication is much less studied.

Researchers say the new findings push back the development of context-free vocal calls to our shared ancestor with bonobos, 6-10 million years ago.

The paper is published in the journal PeerJ.

Similar to babies

For a long time, it was assumed that non-human primates, including great apes like chimpanzees and bonobos, could only communicate using calls that were tied to specific emotional states - such as screaming in alarm, or barking for aggression.

Using a single vocal signal in multiple contexts - referred to as "functional flexibility" - was thought to be a human ability. And it is something we develop very early.

Babies as young as 3-4 months, for example, have been shown to use squeals and growls across a wide range of situations, whether they are happy, distressed or neutral. These sit alongside other noises that are obviously tied to particular emotions, such as crying and laughing.

Dr Zanna Clay from the University of Birmingham noticed a similar phenomenon among the bonobos she was studying in the Democratic Republic of Congo.

As well as the usual screams, barks, pants and grunts, she heard a particular squeaking sound or "peep" being used all over the place.

Image copyright Zanna Clay/Lui Kotale Bonobo Project Image caption Calls were recorded from wild bonobos at the Lui Kotale site in DR Congo

"It seemed very flexible," Dr Clay told BBC News. "In basically every context you can imagine a bonobo experiencing, they peep.

"So we collected recordings of peeps produced in many different contexts."

Working with colleagues from the University of Neuchatel, Switzerland, Dr Clay found that the peeps used in several different positive or neutral situations - such as feeding or travelling - were acoustically identical, just as she had suspected.

This is important because the "meaning" of the peeps must be determined partly from their context - whether that is other calls delivered in a sequence, or other things that the bonobos are doing.

"On their own, [the peeps] don't tie so strongly to one meaning," Dr Clay explained.

So it seems that this kind of "structural flexibility", considered one of the building blocks of human language, is not unique - in fact it turned up several branches back in our family tree.

"Our capacity for this type of flexible signalling was probably a much older capacity than just the human lineage," Dr Clay said.

Power of inference

Dr Simon Townsend studies the evolution of animal communication at the University of Zurich. Commenting on the study, he agreed that it fits into an expanding view of the sophistication of primate "language" - and that bonobos are a somewhat neglected species in this field.

"It's not easy to get access to these animals in the wild... and this is really important data," he told the BBC.

"It goes along with a growing body of evidence that suggests that primates do have quite a bit of control... and goes against the general idea that animals are somehow constrained by their emotional state."

Image copyright Zanna Clay/Lui Kotale Bonobo Project Image caption The bonobos "peep" in various contexts - including grooming, feeding, travelling and resting

Dr Townsend also pointed out the implications for the bonobos' cognitive abilities, in terms of understanding the peeps.

"Non-human primates... seem to have quite good pragmatic inference abilities - and they need these, as a way to differentiate the meaning of the calls."

This is the same ability that humans use, he explained, to figure out whether the word "bow" refers to a knot or a weapon.

"This [study] suggests that something similar might be going on, because the peep produced in the positive context is more or less the same as the peep produced in the neutral context. And therefore, because the receiver can't use acoustic structure to determine meaning, they have to use other cues."

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