A recent investigation led by voice scientist Christian T. Herbst from the University of Vienna and colleagues from Anglia Ruskin University provides new insights into the vocal capabilities of New World monkeys, the group of all original primates of the American continent: They can produce "voice breaks" similar to human yodeling, but support a much wider range of frequencies. The findings are published in the journal Philosophical Transactions of the Royal Society B.

While humans have evolved the ability to speak, our closest relatives in the animal kingdom – apes and monkeys – lack this skill. They do, however, possess specialized anatomical structures in their larynx that humans have lost during evolution: thin, lightweight tissue membranes located at the top edges of their vocal folds. Previous research has suggested that these structures contribute to the complexity of the animals' vocalizations, but their exact role has remained largely unclear – until now, when an international research team has succeeded in uncovering their function.

Revealing the Mystery of Monkey Yodeling

The new research shows that these membranes facilitate abrupt frequency transitions, greatly expanding the range and complexity of monkey vocalizations. Using a combination of methods including non-invasive in vivo recordings, ex vivo laryngeal experiments, and computational modelling, the researchers identified two distinct modes of vocal fold vibration. The first mode, involving only the vocal folds, produces low-frequency sounds similar to human phonation. The second mode, also involving the vocal membranes, results in much higher frequency oscillations, producing dramatic "voice breaks", similar to human yodeling. In some cases, these shifts span more than three octaves – far beyond human capabilities, where voice breaks are typically limited to a single octave.

Evolution and Anatomy Behind Monkey Communication

"This is a fascinating example of how nature provides the means of enriching animal vocalization, despite their lack of language. The production of these intricate vocal patterns is mostly enabled by the way the animals' larynx is anatomically shaped, and does not require complex neural control generated by the brain," explains Christian T. Herbst, lead author of the study from the Department of Behavioral & Cognitive Biology at the University of Vienna.

Jacob D. Dunn from Anglia Ruskin University, senior author of the study, adds: "These results show how monkeys take advantage of a novel evolutionary feature – the vocal membrane – which allows for a wider range of calls to be produced, including these 'ultra-yodels'. This might be particularly important in primates, which have complex social lives and need to communicate in a variety of different ways."

The study further suggests that vocal membranes, while enhancing pitch range, also introduce instability in vocalization. "Our study shows that vocal membranes extend the monkey's pitch range, but also destabilize its voice. They may have been lost during human evolution to promote pitch stability in singing and speech," explains Tecumseh Fitch from the University of Vienna, an expert in human vocal evolution and co-author of the study.This research, conducted by an international team of scientists from Austria, the UK, Japan, Sweden, and Bolivia, provides new perspectives on the evolution of vocal production mechanisms and their role in primate communication.

You can find a "yodel sample" from the study here. A White-fronted capuchin was recorded here.

Contributors include researchers from the Department of Behavioral and Cognitive Biology, University of Vienna, Austria; the Department of Animal and Environmental Biology, Faculty of Science & Technology, Anglia Ruskin University, Cambridge, UK; the Department of Mechanical Engineering, Ritsumeikan University, Japan; the Graduate School of Human Sciences, Osaka University, Japan; the Department of Speech, Music and Hearing, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden; and La Senda Verde Wildlife Sanctuary, Bolivia.