Horse neighing has proven to be a unique vocal phenomenon: the animal produces two sounds simultaneously—a low "voice" (vibration of the vocal cords, like in humans) and a high whistle (turbulent airflow in the larynx, like in rodents).
This phenomenon, known as bifonism, has long puzzled scientists, as large mammals typically cannot whistle. A crucial experiment with helium showed that when regular air was replaced with this gas, the high-frequency component shifted upward (as would occur with a whistle), while the low frequency remained unchanged, which is characteristic of vocal cord vibrations.
Anatomical data confirmed that two different sound production mechanisms are simultaneously involved in one larynx. It is suggested that such complex vocalization allows horses to convey multiple emotional signals in a single sound.
Interestingly, donkeys and zebras lack this high note—indicating that horses acquired it after the species split. When a horse neighs, it is effectively "singing" and "whistling" at the same time, using its larynx as two instruments simultaneously.
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