different frequencies travel through varied environments. In terms of physiology, the larger an animal gets the lower frequency of sound it is able to produce. This is because with an increase in size comes an increase in vocal fold length, and longer vocal folds confer an increased ability to produce lower frequency sounds. Because of this relationship, a bugle can be directed toward other bulls or toward cows to demonstrate the size and thus fitness of the bull vocalizing. A bull will direct his bugle toward his cows while gathering them or while chasing an estrus cow. A herd bull will direct his bugle toward another bull to express his dominance over the herd, while a satellite bull may use his bugle to challenge the herd bull. The reason for the high-frequency portion of the bugle is due to the propagative efficiency of differing frequencies through varying environments. Studies have shown that as a bull's harem increases in diameter, meaning the cows become more dispersed, he tends to vocalize more frequently than if they were within closer proximity. The higher-pitched section of the call propagates through the environment better, which is why the bull uses it to congregate a harem that is becoming more spatially dispersed and thus harder to defend. Acoustic analyses comparing bull elk bugles with cow elk cohesion calls show a notable degree of acoustic similarity, indicating that both vocalizations may perform a congregating function, which is why the bugle is often used by the bull to condense his harem. Bugling physiology 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 biphonetic (two