The ability to produce such a high frequency vocalization by such a large animal is unusual. As explained above, this is because larger body mass positively correlates with longer vocal folds and thus lower frequency emission. Larger body size also corresponds with a decreased ability to emit high frequency vocalizations. Bull elk overcome this by a unique anatomical mechanism that produces sound using a different pathway than the vibrations of the vocal folds. Bull elk constrict their supra laryngeal vocal tract, specifically in the nasal cavity in order to create a smaller opening for exhaled air to pass through. As air moves through this opening it causes the tissues to vibrate and produce the high frequency sound waves which comprise the "whistle" portion of the bugle. This anatomical development for bioacoustics in elk was discovered upon sonographic analysis of bugle vocalizations which revealed a biophonetic (two simultaneous frequencies) display. One frequency was high (the result of the supra laryngeal constriction), and the other was low (the result of normal vocal fold oscillations).