Small adaptation helps lemurs make a big noise

Published: 29 April 2024 at 23:00

Study is first to describe and recreate lemurs' one-of-a-kind vocal structure

New research has discovered that lemurs, the small primates native to Madagascar, are capable of exaggerating their size thanks to the unique structure of their larynx.
Published in the journal Scientific Reports, the study is the first to show that different species of lemurs all possess an anatomical feature unlike any other primate - an additional pair of vocal folds - as well as demonstrate the function of this one-of-a-kind structure.
Co-authored by Dr Jacob Dunn of Anglia Ruskin University, and led by colleagues from Kyoto University in Japan, the study involved carrying out CT scans of the larynges of different species of lemurs and then constructing a silicone model to recreate and test the acoustic properties of these additional vocal folds.
The researchers studied the larynges from two families of lemurs - Lemuridae, which include the ring-tailed lemur (Lemur catta) and black-and-white ruffed lemur (Varecia variegata), and Cheirogaleidae lemurs, such as the grey mouse lemur (Microcebus murinus) - and found that both families possess these unique vestibular vocal folds.
The results suggest that lemurs acquired the twin vocal folds as a novel evolutionary adaptation, and this feature was probably acquired by their common ancestor at least 35 million years ago.
Non-human primates possess a range of different call types and produce these using the same basic principles as speech production in humans. The initial noise is generated by airflow from the lungs, which causes vibrations in the larynx. The amplitude (loudness) and frequency (pitch) of the call is determined by the specific acoustic properties of the larynx and vocal tract.
In a similar way that howler monkeys' enlarged larynx, into which a laryngeal air sac extends, allows them to be such powerful long-distance communicators, the researchers believe lemurs' additional vocal folds have similar practical benefits.
They suggest they are responsible for enriching the lemurs' vocal repertoire, allowing them to make a wider variety of calls, and also making vocalisation more efficient, meaning they expend less energy communicating over distances.
By creating and testing a silicon model of the larynx, the researchers found that the additional vocal folds also lower the frequency of the sounds produced. This makes a lemur sound larger than it actually is, and being able to exaggerate its size may have important benefits, particularly around competition.
Co-author Dr Jacob Dunn, Associate Professor in Evolutionary Biology at Anglia Ruskin University (ARU), said:

Senior author Professor Takeshi Nishimura, of the Center for the Evolutionary Origins of Human Behavior at Kyoto University, said:

The full open access study, published in the journal Scientific Reports, is available here: https://www.nature.com/articles/s41598-024-54172-z