The monkeys that surpass the Alps' yodelers
The world's most advanced yodelers are to be found in the rainforests of Latin America – not in the Alps. New international research, with contributions from KTH Royal Institute of Technology, has discovered that some monkeys have considerable vocal resources. The jungle cry is very useful in the social game.
The secret behind the calls lies in an anatomical discovery: an extra-thin membrane that sits on top of the vocal cords - the so-called vocal membrane. This structure inside monkeys' larynxes, which humans have lost over the course of evolution, acts as an amplifier for the pitch and speed of voice variations.
The monkeys' calls have a technical complexity similar to classic alpine yodeling - but with a twist. Their sonic resources cover a frequency range five times greater than what human yodelers can usually manage. While humans can stretch over one octave, some monkeys can reach over three.
Yodeling plays an important role in the social life of monkeys. The sounds are used to attract attention, make contact and identify each other in the dense greenery of the rainforest, the researchers report.
Sten Ternström, professor of music acoustics at KTH Royal Institute of Technology, has contributed with analyses of electroglottograms - electromechanical signals that show how the vocal cords work.
"Having an extra-thin membrane along the upper edge of the vocal cords is quite common among mammals," he explains.
"Given that the membrane is present, the monkeys’ ability is not so surprising," says Ternström.
It is in an area stretching from Mexico to Argentina that the researchers have found the most advanced yodelers. The monkeys in these regions - including howler monkeys, capuchins, squirrel monkeys and Peruvian spider monkeys – were found to have the largest vocal membranes of all.
The study is based on both computed tomography analyses and simulations, and fieldwork in the Bolivian La Senda Verde Wildlife Sanctuary. There, the monkeys' calls were captured in audio recordings that are now helping researchers understand the evolution of the voice – and its potential in the animal world.
Text: Christer Gummeson ( gummeson@kth.se )