A new study has identified the distinct mechanisms horses employ to produce the simultaneous high- and low-pitched sounds characteristic of their whinnies.
Researchers found that the low-frequency component originates from vocal fold vibrations, while the high-frequency sound is generated by a unique whistling process within the larynx.
This discovery clarifies a long-standing question in animal vocalization and highlights a novel form of sound production in large mammals.
Unraveling the Whinny
The horse whinny, also known as a neigh, is a complex vocalization utilized for various social interactions, including greetings and emotional expressions. It is characterized by the unusual simultaneous occurrence of both high and low frequencies, a phenomenon termed biphonation.
While the low-pitched sound was understood to result from air passing over and vibrating tissues in the voice box, similar to human speech, the origin of the high-pitched component in a large animal remained a subject of scientific inquiry. Large animals typically produce lower-frequency sounds due to their larger vocal systems.
The Mechanism of Dual Sound Production
Research published in the journal Current Biology, led by animal behavioral scientist Dr. Elodie Mandel-Briefer from the University of Copenhagen, details the two distinct methods horses use to create their whinnies. The study found:
- Low-Frequency Sound: This is produced by the vibration of the horses' vocal folds, as air pushes against them, causing them to open and close.
- High-Frequency Sound (Whistle): This sound is generated through a unique whistling mechanism within the larynx. Air vibrates tissues in the voice box while an area just above the vocal folds contracts, creating a small opening. Air forced through this gap creates the whistle. Researchers identified a previously undocumented cavity above the vocal folds, hypothesized to be involved in forming a vortex for this whistle.
This process differs from human whistling, which is typically done with the mouth.
Experimental Evidence
The research team employed multiple experimental approaches to confirm these findings:
- Endoscopic Observation: Small cameras were inserted through the noses of 10 breeding stallions. Observations during whinnies showed vocal folds vibrating for the low-frequency sound and strong cartilage above the larynx constricting to create the small opening for the high-frequency whistle. This visually confirmed the distinct actions for each sound component.
- Excised Larynges: Horse larynges obtained from a butcher were used in a lab setting. Air and helium were blown through these isolated voice boxes while mimicking muscle contractions. Both low and high tones were successfully reproduced. When helium was introduced, the high pitch increased, consistent with whistling behavior, while the low pitch remained unaffected, confirming two distinct sound sources.
- CT Scans: 3D scans of the larynges helped visualize the internal structures, including the cavity above the vocal folds.
- Neurological Study: Stallions with recurrent laryngeal neuropathy, a condition that can paralyze vocal folds, were recorded. These horses exhibited a partially absent low tone but an unaffected high pitch, further supporting that the high pitch is not produced by vocal fold vibration.
Unique Vocal Abilities
Horses are now recognized as the first known large mammals to utilize this laryngeal whistling mechanism.
While some small rodents, such as rats and mice, whistle similarly, horses are unique among all known animals for whistling through their voice boxes while simultaneously producing other vocal sounds. Biphonation is more commonly observed in birds, which possess dual vocal structures, making its occurrence in mammals, with a single set of vocal cords, rare.
Potential Functions and Evolutionary Questions
Researchers propose several theories regarding the evolutionary purpose and communicative functions of the horse whinny:
- Complex Communication: The two-toned whinnies may allow horses to convey multiple messages and express a more complex range of emotions simultaneously during social interactions.
- Distance and Emotional Signaling: The high pitch might serve for long-distance communication due to its louder production, while the low pitch could be for shorter distances. This could explain its use during separations or reunions. Additionally, the high frequency might signal whether an emotion is pleasant or unpleasant, while the low frequency could indicate its intensity.
A significant remaining question concerns the evolutionary development of horses' two-toned calls. Similar vocalizations are observed in wild Przewalski's horses and elks, but not in more distant horse relatives like donkeys and zebras. Further research is needed to fully understand these aspects of horse communication.