Anatomical Observation Of Howler Monkeys

With over 15 different classified species, howler monkeys that are native to Central and South American rainforests are well known for their low pitched but high volume distinctive howls. Big in size yet only weighing 10kg, their distinctive howls can be heard over 3 miles away from their location even in the dense rainforest. Their ability to emit howls is the result of their large hyoid bones and their throat structure that works together to project the loud howls. Various studies have been done to exam their social patterns and if the loud howls they emit are part of a larger scheme in their social behaviors. Howler monkeys not only use their howls to communicate with other monkeys that are in their troop, but they also use their howls for
Schön at the Department of Anatomy in The John Hopkins University School of Medicine was the first study that allowed for dissection of howler monkeys and the observation of the throat and organs in howler monkeys. While the study used anatomical observation and were not conducted in an experimental setting, the study allowed for an in depth explanation. The study used 15 different howler monkeys, comprised of two different species to examine the hyolaryngeal organs and the serial saggital sections of the organ. In the dissection of the monkeys, Schön discovered that howler monkeys had developed a pneumatic sphincter. This was the results of the pharyngolaryngeal sacs acting upon cartilages, and this allowed the monkeys to restrict their laryngeal vestibule and open their thyrohyoid canal. The chambers in the hyoid then can be used as a means of intensifying sounds. Following the complete dissection of the monkeys, Schön proposed that in order for howler monkeys to reach such amplification in their howls, pressure had to be built in a part of a vestibule. The pressure was able to be sustained by the narrowing of the walls in the chamber. When this occurred, a part of the thyrohyoid canal opened and the vibrating air columns entered the hyoidean air sac.(Schön, 1971) This proposed process along with the discovery of a pneumatic sphincter is what allows howler monkeys to emit such loud
Howler monkeys may howl throughout the day for various reasons. Howler monkeys have been observed to howl at dawn between the hours of 5AM to 6AM lasting for as short as four minutes to as long as thirty minutes (Altmann, 1959). With further observation, the howls given in the morning are the same frequency as those given during disputes with other troops. Altmann noted that this may be considered an announcement that the territory they are in belongs to them. They have been also observed howling at disturbances, such as other animals and