Gibbons are soprano singers of the animal world

If you want to hear the clear, piercing voice of a soprano, don't bother with the opera. Just listen to a gibbon. Giving gibbons helium to breathe and studying its effect on their calls provides evidence that the apes call to each other using similar mechanisms to trained human sopranos.

Gibbons are known for their high-pitched "songs", which are unusually melodious for animal calls (listen to a recording).

Wondering how they produce the songs, Takeshi Nishimura of Kyoto University in Aichi, Japan, and colleagues recorded the calls of a captive white-handed gibbon (Hylobates lar) ? first in normal air, and then in air that contained 20 per cent oxygen, 30 per cent nitrogen and 50 per cent helium.

It sounds like a prank, but making an animal inhale helium and measuring how it affects their calls offers clues to how their vocal system works.

It's unclear how most animals produce their calls, but it's thought that in many cases it is the fixed shape of their vocal tract that determines the exact frequency of the sound waves, which make up the call. Because sound waves have a higher frequency in helium than in air, the gas affects the fundamental frequency of these calls.

Unchanged frequency

However, humans can control the frequency of the sound source, and change the shape of their vocal tract to modify it further. One result of this is that the fundamental frequency is not affected when humans breathe helium ? although the resulting sound is altered because the helium changes how the air behaves in the vocal tract, boosting the higher harmonics.

Nishimura's experiment showed that gibbon calls are affected by helium in a similar way to human calls. When the gibbon was breathing helium, the fundamental frequency of her calls was unchanged, suggesting it was produced independently of her vocal tract (listen to a recording).

Gibbons' high-pitched calls are quite simple compared to normal human speech, which uses combinations of different frequencies to produce consonants and vowels. Gibbon calls don't bother with such complexity, but neither do soprano singers. Both sacrifice detail for sheer volume and a purity of tone that allows the call to resonate ? which explains why it's difficult to make out the words when listening to opera.

While other animals such as howler monkeys can produce loud calls that carry for many kilometres, soprano calls are probably unique to gibbons, says Nishimura.

Journal reference: American Journal of Physical Anthropology, DOI: 10.1002/ajpa.22124

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