Compared to humans, most primates produce a limited range of vocalizations: At one end of the spectrum, there’s the Calabar angwantibo, an arboreal west African critter capable of offering up just two distinct calls. At the other end, there’s the bonobo, a skilled chatterbox known to voice at least 38 different calls.
A new study published in Frontiers in Neuroscience suggests these variations can’t be attributed simply to inadequate vocal anatomy. Like their hominid cousins, non-human primates possess a functional larynx and vocal tract. The crux of the matter, according to lead author Jacob Dunn, a zoologist at Anglia Ruskin University in Cambridge, is brainpower.
Dunn and co-author Jeroen Smaers of New York’s Stony Brook University ranked 34 primate species according to vocal ability, as represented by the number of distinct calls the animals produce. The pair then analyzed these rankings in relation to existing studies of the respective species’ brains.
Apes with varied vocalization patterns tended to have larger cortical association areas (neural regions responsible for responding to sensory input) and brainstem nuclei involved in control of the tongue muscles, Victoria Gill reports for BBC News.
These findings, according to a press release, reveal a positive correlation between relative size of cortical association areas and primates’ range of distinct vocalizations. In layman’s terms, speech ability comes down to neural networks, not vocal anatomy. Primates whose sound-producing brain regions are larger can produce a wider variety of calls than those with relatively smaller brain regions.
Dunn and Smaers’ research offers insights on the evolution of speech, Gill notes. Instead of attributing speech skills to humans’ allegedly superior intelligence, the study suggests that speech evolved in conjunction with the rewiring of human brains.
As mankind placed increasing importance on vocal communication, neural regions evolved to fit these needs. Apes, on the other hand, adapted to fit different priorities, retaining an anatomical capacity for vocalization but failing to develop the accompanying neural characteristics needed for speech.
In an interview with Gill, Durham University zoologist Zanna Clay, who was not involved in the study, described the new findings as “interesting,” but added that scientists still lack a basic understanding of how primates use and interpret vocalizations.
Clay, co-author of a 2015 study on bonobo communication, previously told BBC News’ Jonathan Webb that bonobos release identical squeaking sounds, or “peeps,” during disparate situations such as feeding and traveling.
“On their own, [the peeps] don't tie so strongly to one meaning," Clay said.
Within a certain context, however, peeps relay different meanings—perhaps related to the situation at hand or placement in a sequence of vocalizations. This suggests that bonobos are capable of understanding “structural flexibility,” or the use of a single vocal signal in multiple contexts. This phenomenon was previously believed to be a uniquely human ability, Webb writes.
“We do not even really understand how the primates themselves classify their own vocal repertoires,” Clay tells Gill. “This needs to come first before correlations are made. We know that many primates and other animals can escape the constraints of a relatively fixed vocal system by combining calls together in different ways to create different meanings. The extent to which call combinations might map on to [brain anatomy] would be a promising avenue to explore."