Lemurs are able to pretend to be much larger than they really are using only their voice, thanks to a special quirk of evolution, and scientists have discovered how they do it.
These small primates, native to Madagascar, have a unique voice box structure that has a whole extra pair of vocal folds, allowing them to communicate using more complex noises, and increase the volume of their calls to feign being larger in size, according to a new study in the journal Scientific Reports.
This could give them an advantage when competing with each other over territory or mates.
Lemurs are generally social animals and live in groups. Their social structures can be complex, with some species exhibiting matriarchal dominance hierarchies—a rare trait among mammals. They communicate using various vocalizations, scents, and visual signals.
"All primates have a pair of vocal folds in their larynx, which vibrate to modify the air flow, and our research shows that all species of Madagascan lemurs have a unique additional pair of folds in the vestibular region, parallel to the vocal folds," Jacob Dunn, an associate professor in evolutionary biology at Anglia Ruskin University in the U.K., said in a statement.
The researchers, from Anglia Ruskin University and Kyoto University in Japan, took CT scans of the voice boxes of lemurs from two families: lemuridae, including the ring-tailed lemur (Lemur catta) and black-and-white ruffed lemur (Varecia variegata), and cheirogaleidae lemurs, including the gray mouse lemur. They found that all of these species had the extra set of vocal folds present in the larynx.
They then created silicone models of the larynxes of lemurs, which they used to test their acoustic properties. The results showed that lemurs with these extra folds were able to call louder and include more complicated noises in these calls than those without.
"Primates use varied vocal repertoires to communicate, and the larynx plays a key role in generating these calls," study co-author Takeshi Nishimura, a professor at Kyoto University, said in the statement.
"By recreating the lemurs' unique vocal structure through our model and studying its oscillation properties, we found that simultaneous vibrations of the twin pairs of vocal folds lowers the frequency of the calls produced—helping the lemur sound larger than it is—and also serves to improve vocal efficiency."
The researchers suggest that these extra vocal folds likely evolved as an adaptation around 35 million years ago, allowing them to communicate more efficiently and expend less energy to call further distances. This adaptation may also have helped the lemurs make themselves sound larger than they really are by lowering the frequency of their calls, which may be advantageous in mating.
"This is likely to have developed because ancestors possessing these twin vocal folds may have had a selective advantage compared with members of the same species without this adaptation," Dunn said.
"We think these twin vocal folds serve to not only increase the complexity of their calls, but, crucially, may also help a lemur exaggerate its body size, which would have significant benefits when competing for territory or mates."
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