Apes and monkeys possess special anatomical structures in their throats called vocal membranes, which disappeared from humans through evolution to allow for more stable speech. However, the exact benefit these provide to non-human primates had previously been unclear. New research provides significant new insights into the diverse vocal sounds of non-human primates, and reveals for the first time how certain calls are produced.
New World monkeys, whose range stretches from Mexico to Argentina, were found to have evolved the largest vocal membranes of all the primates, suggesting these thin ribbons of tissue play a particularly important role in their vocal production and repertoire of calls. This image shows black and gold howler monkeys (Alouatta caraya). Image credit: Jacob Dunn, Anglia Ruskin University.
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.
Led by Anglia Ruskin University and the University of Vienna researchers, the new study shows that these membranes facilitate abrupt frequency transitions, greatly expanding the range and complexity of monkey vocalizations.
The scientists recorded and studied the calls of various primate species at La Senda Verde Wildlife Sanctuary in Bolivia, including the black and gold howler monkey (Alouatta caraya), tufted capuchin (Sapajus apella), black-capped squirrel monkey (Saimiri boliviensis), and Peruvian spider monkey (Ateles chamek).
Using a combination of methods, they 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.
“This is a fascinating example of how nature provides the means of enriching animal vocalization, despite their lack of language,” said Dr. Christian Herbst, a researcher at the University of Vienna.
“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.”
“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,” said Dr. Jacob Dunn, a researcher at Anglia Ruskin University.
“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,” said Dr. Tecumseh Fitch, a researcher at the University of Vienna.
“They may have been lost during human evolution to promote pitch stability in singing and speech.”
The study was published in the journal Philosophical Transactions of the Royal Society B.
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Christian T. Herbst et al. 2025. ‘Monkey yodels’ – frequency jumps in New World monkey vocalizations greatly surpass human vocal register transitions. Phil. Trans. R. Soc. B 380 (1923): 20240005; doi: 10.1098/rstb.2024.0005
