Diana monkeys possess a complex vocal tract whose shape can be adjusted to articulate sophisticated sounds - just as humans do, scientists report.
Diana monkeys' vocal tracts may shed light on primate evolution
Non-human primates were thought to have vocal tracts resembling simple tubes incapable of sophisticated articulation.
But a British-US-German team reports in the Journal of Human Evolution that the alarm calls of Diana monkeys would be impossible without a complex tract.
It says the finding may shed light on how and when human speech evolved.
It is possible that some of the proto-structures in the throat required for talking were already present in our primate ancestors millions of years ago, the researchers argue.
The latest results support acoustic analysis showing Diana monkey calls could not be produced by a simple vocal tract.
But this does not mean the animals are capable of anything more complex than the calls they produce in the wild.
The work was carried out by scientists at the University of St Andrews, UK; Humboldt University in Berlin, Germany; and the Baltimore Zoo in Maryland, US.
The researchers claim it is the first time both acoustic and anatomical data have supported the presence of a non-uniform vocal tract in a non-human primate.
"Previous models of animal sound production always assumed the vocal tract was like a uniform tube. This explained why a lot of animal vocalisations had a very simple acoustic structure," said Dr Klaus Zuberbühler, from St Andrews.
"The human vocal tract is different because it consists of various tubes of different sizes. This explains how we can produce such a range of different sounds that are important for speech."
The team X-rayed the vocal tracts of three adult Diana monkeys (two male, one female) from Baltimore Zoo and dissected the carcass of another which had died at the zoo.
They used the results to generate a computer model that simulates the patterns observed in the vocalisations of Diana monkeys.
"With Diana monkey calls, it is clear there must be at least three different tubes to be able to produce the acoustic patterns they make," said Dr Zuberbühler.
"The suggestion is that they can change the size of these tubes to produce the sounds they do. This is in some senses equivalent to how speech production takes place [in humans]."
These tubes are thought to be formed by constrictions at different sections along the monkeys' vocal tract.
Dr John Coleman, of the phonetics laboratory at Oxford University, UK, said he was not surprised by the findings.
"Just looking at the human vocal tract and other animals, it's quite frequent for non-human primates to have quite complex vocal tracts. Chimpanzees in particular have more complex vocal tracts than human beings," he said.
Dr Zuberbühler said his team was now investigating whether the case found in Diana monkeys was also true for chimpanzees, our closest animal relatives.