It is a charming scene in The Sound of Music when Maria and the Von Trapp children sing about a lonely goatherd yodelling in the Austrian Alps (lay-ee-odl-lay-ee-odl-lay-hee-hoo).
But little did these characters know, their yodelling would be put to shame by monkeys in the rainforests of Latin America.
That’s according to a new study by researchers at Anglia Ruskin University (ARU) and the University of Vienna, collaborating with experts in Japan, Sweden and Bolivia.
The scientists recorded and studied the calls of black and gold howler monkeys, tufted capuchins, black-capped squirrel monkeys and Peruvian spider monkeys in La Senda Verde Wildlife Sanctuary, Bolivia, finding their calls include voice breaks similar to Alpine yodelling.
Except, while human yodellers leap between notes spanning an octave or less, these primates can jump more than three musical octaves at once – in terms of frequency, that's a five-times bigger difference in pitch.
Senior author Dr Jacob Dunn, Associate Professor in Evolutionary Biology at ARU, said these vocal leaps might enable primates to better communicate, as they navigate their complex social lives.
“It’s highly likely this has evolved to enrich the animals’ call repertoire, and is potentially used for attention-grabbing changes, call diversification, or identifying themselves,” he said.
Dubbed ‘ultra-yodels,’ these jumps are produced by strange anatomical structures in the monkeys’ throats, called vocal membranes: extremely thin ribbons of tissue that sit above the vocal folds in their larynxes (voice boxes).
Humans don’t have these vocal membranes. The scientists think they disappeared through evolution, enabling us to speak with more precision.
“Our study shows that vocal membranes extend the monkey’s pitch range, but also destabilise its voice,” said Professor Tecumseh Fitch, expert in human vocal evolution from the University of Vienna and co-author of the study.
While humans yodel by switching between the highest and lowest registers of our voices, monkeys yodel by switching between using their vocal folds – like humans – and these vocal membranes to make their calls.
“This is a fascinating example of how nature provides the means of enriching animal vocalisation, despite their lack of language,” said lead author Dr Christian T Herbst, of the Department of Behavioural and Cognitive Biology 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.”
But not all monkeys are equally good at yodelling. It’s Latin American monkeys, found in treetops from Mexico to Argentina, that the scientists found had the largest vocal membranes of all the primates, suggesting these calls are especially important for these species.
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