Researchers examining the brains and vocal repertoires of primates have offered important insight into the evolution of human speech.
However, relative to humans, non-human primates produce an extremely limited range of vocalisations.
Published in the journal Frontiers in Neuroscience, the research investigated whether the reason primates are incapable of producing speech is because they lack the brain mechanisms needed to control and coordinate vocal production.
The study focused on two particular features of the brain: the cortical association areas that govern voluntary control over behaviour; and the brainstem nuclei that are involved in the neural control of muscles responsible for vocal production.
The researchers, including those from Stony Brook University, found a positive correlation between the relative size of cortical association areas and the size of the vocal repertoire of primates, which can range from just two call types in pottos to at least 38 different calls made by bonobos.
"This study shows, for the first time, a significant positive correlation between the vocal repertoire and the relative size of the parts of the brain responsible for voluntary control over behaviour," said Jacob Dunn, Senior Lecturer at Anglia Ruskin University.
"Cortical association areas are found within the neocortex and are key to the higher cognitive processing capacities considered to be the foundation for the complex forms of behaviour observed in primates.
"Interestingly, the overall size of the primate's brain was not linked to the vocal repertoire of that species, only the relative size of these specific areas," said Dunn.
"We also found a positive relationship between the relative volumes of the cortical association areas and the hypoglossal nucleus in apes, both of which are significantly bigger in these species," he said.
The hypoglossal nucleus is associated with the cranial nerve that controls the muscles of the tongue, thus suggesting increased voluntary control over the tongue in our closest relatives.
"By understanding the nature of the relationship between vocal complexity and brain architecture across non-human primates, we hope we are beginning to identify some of the key elements underlying the evolution of human speech," Dunn said.
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