A bugle is a vocalization made exclusively by bulls. The typical bugle consists of three acoustic parts, a low frequency "on-glide" that sounds guttural in tone, which then ascends into the highest frequency part of the call termed the "whistle", and the last portion of the call, the "off-glide" that returns to a low frequency tone. The function behind this acoustic structure of the bugle is directly correlated to the male's physiology and how 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.