Researchers from Kyoto University recorded 20 calls made by gibbons in a zoo, and compared them with 37 calls made when the animals had breathed in helium.
In normal air the lowest frequencies of a gibbon's song are the loudest, but under the effect of helium the tuning of their vocal cords and resonance of the vocal tract changed to amplify higher frequencies.
The results showed that gibbons are capable of consciously manipulating their vocal chords to make their signature calls, the researchers explained in the American Journal of Physical Anthropology.
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