The larynx is significantly larger relative to body size in primates than in other mammals and also has greater variation, according to a new study.

That’s significant because the “voice box” is considered a key area of evolution, particularly in species with highly developed vocal communication systems. (Its other main functions are protecting the airway during feeding and regulating the supply of air to the lungs.)

In their study, a team led by Anglia Ruskin University, UK, Stanford University, US, and the University of Vienna, Austria, made CT scans of specimens from 55 different species of primates and carnivorans.

The former ranged from a pygmy marmoset (Cebuella pygmaea), weighing just 110 grams, to a Western gorilla (Gorilla gorilla) at around 120 kilograms. The latter spanned a 280-gram common dwarf mongoose (Helogale parvula) to a 180-kilogram tiger (Panthera tigris).

From the scans, the team then produced 3D computer models of the larynges, which were analysed alongside detailed measurements, including body length and body mass.

They found that, for a given body length, primate larynges are on average 38% larger than those of carnivorans, and that the rate of larynx evolution is faster in these species.

There is also more variation in larynx size relative to body size among primates, indicating that primates have greater flexibility to evolve in different ways. Carnivorans follow more of a fixed larynx-size to body-size ratio.

Larynx size was also found to be a good predictor of the call frequency of a species, which demonstrates the relevance for vocal communication of the observed size variations.

“Our study… shows that differences in larynx size predict changes in voice pitch, highlighting the larynx’s crucial role in vocal communication. This is demonstrated by the rich and varied calls produced by many primate species,” says Stanford’s Daniel Bowling, co-lead author of a paper in the journal PLOS Biology.

“The results imply fundamental differences between primates and carnivorans in the forces constraining larynx size, as well as highlighting an evolutionary flexibility in primates that may help explain why they have developed complex and diverse uses of the vocal organ for communication.”