It is the year of the fire monkey! Let’s celebrate with some magnificent screaming monkeys from Central and South America. These monkeys are loud–really loud. They are louder than Rush Limbaugh or heavy machinery–so loud that, in fact, that they are generally regarded as the loudest of all land animals. I am talking, of course, about the howler monkeys. These fifteen species make up the genus Alouatta (which lies within the family Atelidae ). They are new world monkeys ranging from the top of Central America down through South America to Uruguay.
Howler monkeys have short snouts with keen noses (they are capable of smelling their favorite fruits and leaves from 2km away). Depending on the species and gender they range from 56 to 92 cm (22 to 36 in) in height or length…or whatever primary dimension you attribute to monkeys. This measurement does not include their tails which can be up to 5 times the length of their body. They weigh 7 to 10 kg (15 to 22 lbs) and live up to 20 years. Howler monkeys are folivore–they mostly eat leaves. This diet is widely available but it is hard to digest–which means howler monkeys are larger and slower than other New World monkeys (although they supplement their diets with fruit and eggs when they can).
The hyoid bones of howler monkeys are pneumatized–which is to say that the u-shaped bone in the monkey’s neck contains air. Outside of the dinosaurs and their descendants, pneumatized bones are exceedingly rare. The hyoid bone anchors the tongue and the larynx and allows for vocalization. The fact that it is specialized in howler monkeys is one of the factors which allows them to vocalize with such ferocious power. There is an inverse relation between the size of the hyoid bone and the size of the male’s testes. This seemingly random fact is actually a key factor in howler society.
Howler monkey lifestyle diverge into two very different ways of living (except for mantled howler monkeys which live together in large groups and behave somewhat differently than the other 14 species). In one model, a male, who has a larger hyoid, and smaller testes gathers a group of females together with his majestic singing (screaming?) voice and he mates with them exclusively like a sultan with his harem. In the other model several males mate with a group of several females. Seemingly, this free love model requires less loud singing and more gonad mass.
At this point you are probably wondering a great deal about the howler monkey song. What is this primal howl which female monkeys prefer over carnal joy? Musical enthusiasts have compared the baritone shout of the male monkeys to a Gregorian chant, a monstrous belch, or a demonic snowplow. All of these comparisons have some validity…but the sound is so much richer than that. Why don’t you have a listen (at 1:23) and let us what you think?