Howler monkeys, New World monkeys, are known for their extraordinary vocalizations. They produce some of the loudest sounds among land animals, a defining characteristic of their rainforest presence. Their vocal abilities allow communication across vast distances, shaping social interactions and territorial behaviors.
The Distinctive Howl
Howler monkeys produce a deep, guttural roar that can waver in intensity and resonate significantly through the rainforest canopy. Beyond roars, they also produce throaty coughs, gurgling grunts, barks, and woofs. Some listeners describe the sound as akin to a “garbage disposal” or “death metal,” highlighting its raw and powerful nature. These vocalizations can reach up to 140 decibels, greatly exceeding the 70 decibels of a human shout.
Their calls can be heard for several kilometers through dense forest. Reports indicate their howls are audible from as far as three miles (approximately 4.8 kilometers) away. Howler monkey vocalizations are most common at dawn and dusk, signaling peak activity periods. They can also howl at other times, particularly in response to disturbances or perceived threats.
Why Howler Monkeys Howl
Howler monkeys use their powerful vocalizations for various functions within their social and ecological contexts. A primary purpose is territorial defense, where groups announce their presence to warn rival groups and mark boundaries without direct physical confrontation. This acoustic signaling helps maintain spacing between groups, reducing overlap in feeding areas and conserving energy.
Their calls also serve as warning signals, alerting the group to potential predators. Within the group, less intense vocalizations contribute to communication, coordinating movements, and helping members locate one another. Male howler monkeys also use their calls to attract females, acting as a form of sexual advertisement. Additionally, howls can communicate the availability of food resources, such as fruit trees.
Anatomical Adaptations for Sound Production
The vocal capabilities of howler monkeys are possible due to unique biological adaptations, primarily an enlarged hyoid bone. This U-shaped bone, located in the throat, is significantly larger and more cup-like in howler monkeys compared to other primates. It is hollow and forms a large resonating chamber that amplifies and deepens the pitch of their calls. This specialized structure works with their vocal cords and an unusually large larynx and throat, which can balloon outward during vocalization.
Male howler monkeys typically have a larger hyoid bone than females, contributing to their deeper and louder calls. In some species, male howler monkey hyoids can be up to five times larger than those of females. This anatomical specialization allows them to produce sounds that, despite their relatively small body size, can rival the roars of much larger animals.