Deep in the rainforest, the monkeys are yodeling. Their wild calls echo across the foliage, sending signals of sex and survival. For decades, scientists have studied why they make these sounds, but are just beginning to understand how.
A new study asks how monkeys make calls with abrupt frequency jumps, which sound like human yodeling.
Scientists were curious about the role of vocal membranes, thin ribbons of tissue that sit above monkeys’ vocal folds. Humans have vocal folds (vocal cords), but monkeys have both folds and membranes.
“We knew about these vocal membranes,” Jacob Dunn, an evolutionary biologist at Anglia Ruskin University in the U.K., who co-authored the study, told Mongabay. “We knew that they were in all the primates that we studied, but nobody had really studied their actual function.”
To answer this question, researchers used a combination of acoustic recordings, electroglottographs (EEGs) — which measure contact area between vocal folds during use — and modeling. They confirmed that voice production in monkeys from the Americas is made possible by two sets of vocal structures, allowing the monkeys to jump back and forth between frequencies.
Vocal folds produce low-frequency sounds, and vocal membranes create higher tones. Switching between the two produces an effect similar to yodeling. However, monkeys have a much wider range and can oscillate between notes much more rapidly than human yodelers, who rely solely on vocal cords.
A tufted capuchin (Sapajus apella) call in real time and slowed down, with Dr. Christian Herbst of the University of Vienna explaining the frequency jumps made during the call.
“In one extreme case, we recorded a frequency jump of 12 times — that’s 43 semi-tones or about three and a half octaves,” Dunn said. “Humans rarely exceed one octave when yodeling.”
Dunn said this behavior is probably a result of monkeys’ complex social lives in the rainforest. “Having a more complex repertoire of sounds that you can make can be useful if you’re a social species with quite a lot of relationships that you need to manage,” he said.
“This phenomenon,” Dunn told Mongabay, is, “likely to have this role in attention grabbing. A crying baby is a really obvious example of a really non-linear voice. We all know that babies are really annoying, and for good reason — they’ve evolved to be really annoying.”
Most of the research happened at La Senda Verde, a wildlife refuge run a nonprofit organization in the tropical region east of the Bolivian Andes. La Senda Verde works with local and national governments to take in animals from the illegal trade. Having grown up with humans, these monkeys are good candidates for interaction with researchers.
Researchers recorded vocalizations of hand-reared monkeys in the sanctuary, including black-and-gold howler monkeys (Alouatta caraya), red howler monkeys (Alouatta sara), Peruvian spider monkeys (Ateles chamek), black-capped squirrel monkeys (Saimiri boliviensis), tufted capuchins (Sapajus apella), black-handed spider monkeys (Ateles geoffroyi), and white-fronted capuchins (Cebus albifrons).
They also attached small, noninvasive sensors to the throats of tufted capuchins, black-handed spider monkeys and white-fronted capuchins. Attached to an EEG machine, these sensors help visualize the movements of the vocal folds by measuring how much electricity flows across the larynx. When the vocal folds touch, the larynx conducts electricity better.
Researchers also conducted experiments with larynges from two male tufted capuchin monkeys that veterinarians had euthanized for unrelated medical reasons at the Primate Research Institute in Kyoto, Japan. They mounted each larynx on a tube and blew heated air through it while recording sounds and high-speed video of the vibrating tissues. Using sutures attached to motors to control the position of the vocal membranes precisely, they discovered that pulling these membranes to the side caused only the vocal folds to vibrate, producing low sounds. Positioning them centrally made both structures vibrate together, creating high sounds.
To confirm their findings, scientists created a computer model that simulated the same vocal anatomy, demonstrating that monkeys produce their dramatic frequency jumps by switching between these two modes of vibration, something impossible for humans who lack these specialized membranes.
The evolutionary loss of vocal membranes in humans has stabilized human speech. Or, in other words, humans gave up vocal gymnastics for clear communication.
“Vocal membranes extend the monkey’s pitch range, but also destabilise its voice,” co-author Tecumseh Fitch, from the University of Vienna, said in a statement. “They may have been lost during human evolution to promote pitch stability in singing and speech.”
“Human speech is basically determined by a very periodic [regular] signal,” Dunn told Mongabay. “If we were to scream speech to each other, we wouldn’t understand each other.”
Grammy-winning singer-songwriter and vocal coach Melody Walker, who wasn’t involved in the research, compared these findings to advances in understanding the human voice. “So much has changed in [human] vocal science just in the last couple decades,” she told Mongabay. “We’re still learning about the human voice.”
Walker noted that yodeling and similar vocal techniques have sprung up in mountain cultures worldwide and that singing is ubiquitous across humanity.
“Perhaps singing — where vocal registers are amply used,” the study authors conclude, “still enables us to exploit the acoustic richness of our primate heritage.”
Banner image of Black and gold howler monkeys (Alouatta caraya) by Dr Jacob Dunn, Anglia Ruskin University
Liz Kimbrough is a staff writer for Mongabay and holds a Ph.D. in Ecology and Evolutionary Biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here.
Citation:
Herbst, C. T., Tokuda, I. T., Nishimura, T., Ternström, S., Ossio, V., Levy, M., … Dunn, J. C. (2025). ‘Monkey yodels’—frequency jumps in New World monkey vocalizations greatly surpass human vocal register transitions. Philosophical Transactions B, 380(1923), 20240005. doi:10.1098/rstb.2024.0005
FEEDBACK: Use this form to send a message directly to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.
