Primate voice boxes are bigger, evolve at a faster pace, study says

Aug. 11 (UPI) -- The size of the larynx, or voice box, in primates is larger and exhibits greater variation than carnivores, according to a study published Tuesday in the journal PLOS Biology.

The finding suggests the diversification of the larynx was key to the development of vocal communication systems among primates, researchers say.

The larynx serves three main purposes -- protecting the airway during feeding, supplying air to the lungs and enabling vocalization. Evolutionary biologists have long suspected that the larynx was key to the development of vocal communication.

"We found that the larynx of primates is significantly larger relative to body size, has greater variation, and is under faster rates of evolution than in other mammals, which suggests that there is something interesting about the evolution of the larynx in our own order of mammals," co-lead study author Jacob Dunn told UPI.

For the study, scientists analyzed 3D models of the larynx, developed from CT scans, to determine larynx size and its relationship to body size among 55 mammal species.

Researchers looked at the voice boxes of a range of different sized primates, from the tiny pygmy marmoset to the giant Western gorilla. Scientists also studied the voice boxes of both smaller and larger carnivores, from the cat-sized dwarf mongoose to the brawny tiger.

On average, primate species boasted voice boxes that were 38 percent larger than comparable carnivore species. The research also showed a stronger relationship between the size of the voice box and body size in carnivores.

Researchers aren't sure exactly why primates have developed bigger, more evolutionarily flexible voice boxes, but they have a few theories.

Dunn suggests it's possible primates evolved bigger larynges to produce lower frequency sounds, which are better for vocalizing in dense forests. It's also possible that social structures and mating competition played a role.

"We are not convinced that any one of these hypotheses alone is enough to explain the differences we find," said Dunn, reader in evolutionary biology at Anglia Ruskin University in Britain.

"One thing that seems certain is that the greater variation and faster evolution of the primate larynx has meant that primates have been able to evolve a diverse range of uses of the larynx, and this evolutionary flexibility may help explain why we and other primates have developed complex and diverse uses of the vocal organ for communication," he said.

Dunn and his colleagues are currently collecting additional data on the size and structure of voice boxes among other groups of mammals for comparison to the larynges of primates.

"If primates continue to show remarkable differences when compared to several more mammal groups, then this would suggest even more strongly that the primate larynx really is something special," Dunn said.

"We continue to develop new methods to study the larynx in detail, and will also be focusing on other aspects of larynx evolution beyond simply the size of the larynx," he said.