The high fundamental frequency in horse whinnies is generated by an aerodynamic whistle.

Overview

• This research investigates why horses produce whinnies with two distinct fundamental frequencies—one low and one exceptionally high—focusing on the biomechanical origin of the high-frequency component.
• It reveals that the high-frequency sound is produced by an aerodynamic whistle mechanism within the larynx, rather than by traditional vocal fold vibrations, explaining how horses can generate two separate frequencies simultaneously (biphonation).

Background and Motivation

• Mammalian vocalizations are usually predicted by body size: larger animals typically produce lower frequency sounds due to their vocal anatomy.
• Horses, despite being large terrestrial mammals, produce whinnies that include a surprisingly high fundamental frequency (>1,000 Hz), alongside a lower fundamental frequency (~200 Hz).
• The lower frequency is understood to arise from vocal fold vibrations, but the mechanism for the higher frequency was previously unknown.
• Understanding this phenomenon helps explain how diverse and complex vocal behaviors evolved in mammals.

Research Methods

• The study used a combination of in vivo (within live animals) and ex vivo (outside the living body) techniques to investigate horse vocal production.
• Excised larynx experiments were conducted where the larynx was isolated and manipulated; helium was used to evaluate changes in sound production, as helium affects aerodynamic properties and sound velocity.
• Computed tomography (CT) scans provided detailed anatomical images of the larynx structure.
• Endoscopic examinations allowed direct visualization of the larynx during vocalization.
• Acoustic analyses were conducted on horses with recurrent laryngeal neuropathy—a condition affecting nerve control of the larynx—to observe alterations in sound production correlated with nerve damage.

Key Findings

• The high fundamental frequency in horse whinnies is produced via an aerodynamic whistle mechanism within the larynx rather than by the vibration of vocal fold tissues.
• Evidence from helium tests and anatomical analyses showed that the high-frequency sound characteristics matched those expected from a whistle, where airflow generates vibrations by passing through a particular constriction or structure in the larynx.
• CT scans and endoscopy identified laryngeal adaptations that facilitate this aerodynamic whistle production.
• Horses with nerve damage that disrupts vocal fold function still produced the high-frequency whistle component, indicating independent control and generation from traditional vocal fold vibration.
• The simultaneous presence of these two separate sound sources explains biphonation—where two fundamental frequencies are produced simultaneously within a single vocalization.

Implications and Conclusions

• This dual-frequency production likely serves an adaptive function by allowing horses to convey multiple messages simultaneously, potentially enhancing communication efficiency and social interactions.
• The finding highlights the importance of both anatomical specializations and aerodynamic mechanisms in expanding the complexity of vocal communication across species.
• Such mechanisms may be more widespread in mammals than previously recognized and contribute significantly to vocal diversity.
• The research expands our understanding of mammalian vocal production beyond the classical view of vocal fold vibrations as the sole sound source.