The typical neighing of horses contains two fundamental frequencies emitted simultaneously: one low and one surprisingly high. A new study led by Romain Adrien Lefèvre and published on Current Biology this year, shows that these two components have different origins. The lowest frequency arises from vibration of the vocal cordsas occurs in most mammalian sounds. The highest one, however, is generated by a aerodynamic mechanism similar to a whistle produced by the flow of air in the larynx. The research combines laboratory experiments on isolated larynges, anatomical scans, endoscopic recordings in live horses and analyzes of animals with laryngeal pathology, showing how these two systems work together to produce the characteristic double sound.
Horse neighing has two different frequencies and sounds
When a horse neighs it often produces two frequencies at the same time. In bioacoustics this phenomenon is called biphony: means that they appear in the same sound two independent fundamental frequencies. There fundamental frequency it is the basic pitch of a sound, i.e. the main oscillation that determines how low or high it is. In horse neighing there are:
- a higher frequency lowabout 200–400 Hz
- a much higher frequency highOften over 1000 Hzup to 1500 Hz
This combination is curious because horses are large animals. Generally, in larger animals the voice is lower: It’s a called relationship acoustic allometrywhich links body size and voice frequency. However, horses produce much higher-pitched sound components than would be expected from a mammal of approximately 500kgwhich should make sounds less than 100Hz. To understand where this unusually high frequency comes from, the researchers designed a series of experiments.
The new study on the sound of horses
The first step was to directly study the organ that produces the sound: the larynx. The researchers analyzed six horse larynxes in the laboratory, passing air through the tissues to artificially recreate the sounds produced during whinnying. Then they repeated the experiment using helium instead of air. This step is fundamental to distinguish two possible mechanisms: if the sound is produced by vibration of the vocal cordsthe frequency does not change when going from air to helium. If the sound is produced by a aerodynamic whistlethe frequency increases, because the speed of sound changes in less dense gases.
The results were very clear:
- the sounds at low frequency they didn’t change when you switched from air to helium
- the sounds at high frequency increased significantly when helium was used
This behavior is typical of aerodynamic whistles and indicates that the acute component of nitrite does not arise from tissue vibration.
To understand whether the vocal cords could still produce such high frequencies, the researchers analyzed the anatomy of the larynx with computed tomography (CT) on three larynges. The average length of the vocal cords was approximately 24mm. According to biomechanical models of the voice, vocal folds of this size could generate frequencies between approximately 24Hz with low voltage e 400 Hz with very high voltage. This interval coincides with the severe component of nitrite, but cannot explain the frequencies of approximately 1500 Hztypical of the acute component. To produce such high sounds through vibration would require tissue tension and pressure greater than 5 MPafar beyond the normal physiological values of mammalian vocal cords. Even anatomy therefore suggests that the high-pitched sound has another origin.
The authors then recorded with an endoscope the movements of the larynx in 10 stallions neighing. The images show a precise sequence: at the beginning of the nitrite, the cartilages of the larynx approach and narrow the glottis. This narrowing produces a fast air blastgenerating the high-pitched sound. Subsequently the vocal cords also begin to vibrate, producing the low component. In practice, the two sounds arise from two different but simultaneous mechanisms.
To definitively test the hypothesis, researchers studied horses with recurrent laryngeal neuropathy (RLN)a pathology that can paralyze one of the vocal cords. If both frequencies depended on the vocal cords, both should be altered by the disease. It actually happened that frequency low she was often disturbed or absent and the high frequency remained normal. This result confirms that the acute component does not depend on the vocal cords.
A whistle behind the larynx: why nitrite has two frequencies
By combining all the data, the researchers proposed a mechanism by which air is pushed through a narrow opening in the larynx, the flow forms a turbulent jet, this produces a sound similar to a aerodynamic whistle. Structures such as small cavities of the larynx could function from resonance chambersstabilizing the sound, a bit like what happens when a person whistles with their lips.
Biphony might have advantages in communication. According to the authors, the two frequencies could convey different information at the same time: the frequency low could indicate characteristics of the animalsuch as body size; that high could transmit emotional states or urgency of the signal. Furthermore, very high-pitched sounds may be more noticeable or more audible at a distancemaking nitrite more effective as social signal.
