The howler monkey, belonging to the genus Alouatta, is one of the largest primates found in the New World, inhabiting the canopies of Central and South American forests from southern Mexico to northern Argentina. While largely folivorous, or leaf-eaters, they are famous for their extraordinary vocalizations. The impressive sound they produce is considered one of the loudest among all land animals, earning the monkeys their descriptive name.
The Decibel Level of the Howl
The sound pressure level produced by a howling male often reaches 90 decibels (dB) when measured at a distance. This volume is comparable to the noise of a lawnmower or a jackhammer operating nearby. At close range, however, the intensity of their guttural roar can peak at an astonishing 140 dB.
A human shouting reaches only about 70 dB, and sounds above 120 dB can cause immediate physical discomfort. The maximum volume of the howler monkey’s call approaches the sound level of a jet engine taking off (120 to 150 dB). This sheer power allows the calls to carry over vast distances through the dense, sound-absorbing environment of the rainforest.
The thunderous cries can travel as far as three miles (five kilometers) in the jungle under optimal conditions. This impressive range means a single monkey can project its presence across a significant portion of the forest. The volume and low frequency of the call are highly adapted to the environment, enabling the sound waves to penetrate the thick vegetation without scattering.
The Biological Mechanism of Sound Production
The ability of this primate to generate such a booming sound is due to a unique biological specialization in its throat. The key anatomical feature is an enlarged, highly modified hyoid bone, a U-shaped bone that supports the tongue in most mammals. In the howler monkey, this bone is significantly inflated and hollowed out.
This modified bone structure, which is cup-like and contains large air sacs, functions as a sophisticated resonating chamber. As the monkey forces air through its larynx, sound waves enter this hollow hyoid structure. The bone then amplifies the sound and significantly lowers its pitch, transforming vocal cord vibration into a deep, reverberating roar.
Male howler monkeys exhibit hyoid bones up to five times larger than those of females, directly correlating with their louder and deeper calls. The size of the larynx and the surrounding throat musculature are also enlarged to accommodate this impressive vocal apparatus. This energy-intensive adaptation makes the species one of the loudest land mammals on the planet.
The Purpose and Range of the Monkey’s Call
The primary function of the loud howl is ecological and social, revolving around resource management and group spacing. These vocal displays are most frequently heard during the morning and evening, functioning like a “dawn chorus” to announce a group’s location and territory. This acoustic signaling allows rival groups to maintain a safe distance, minimizing the energy cost and danger of physical conflict.
The calls are also associated with the defense of preferred food sources, especially when groups are feeding on major fruit trees. By broadcasting their presence, a group can effectively deter intruders from entering a desirable patch of resources. The calls serve as an advertisement of occupancy and a claim to a specific area of the canopy.
For males, the howl plays a significant role in sexual selection and mating competition. The size of the hyoid bone determines the depth and perceived power of the call. Studies suggest an evolutionary trade-off exists between investment in a large, loud hyoid and investment in large testes.
Species where males live in one-male, multi-female groups tend to have larger hyoid bones and smaller testes, using the powerful call to scare off rivals and attract females. Conversely, species living in multi-male groups, where competition occurs after mating, possess smaller hyoids but larger testes to maximize sperm competition.