Mammals
Howler Monkey
Alouatta · also called howler monkey, mono aullador, guariba
The howler monkey is the sound of a Central American dawn — a deep, rolling roar that comes out of the dark canopy and carries for kilometres. It is the loudest land animal in the Americas, and it produces that noise with a throat bone hollowed into a natural megaphone. The rest of the day, it barely moves.
Overview
Howler monkeys live in forests from southern Mexico to northern Argentina. They are leaf-eaters above all — young leaves, plus fruit and flowers — fermenting the low-energy food in an enlarged gut. They are 4–10 kg, stocky, bearded, with a strongly prehensile tail that has a bare grippy pad on the underside.
The roar
Howlers have a hugely enlarged hyoid bone in the throat, hollowed into a shell-like resonating chamber, plus an enlarged voice box. The result is a deep roar that carries 3–5 km — among the loudest calls of any animal.
Talking instead of fighting
Troops roar at dawn and dusk and when they meet, mainly to advertise their location so groups can avoid each other and settle boundaries by voice rather than combat.
Built for stillness
Running on leaves, a howler rests and sleeps 15–20 hours a day, moving slowly and economically — one of the least active monkeys. All howlers also have full three-colour vision, unusual for a New World monkey, likely to spot reddish young leaves.
Status
Howler monkeys are Least Concern overall but declining, with several threatened species. They are very vulnerable to yellow fever outbreaks and to forest fragmentation that isolates troops.
Howler Monkey: common questions
How loud is a howler monkey, and how does it make the sound?
A chorus of howler monkeys can reach around 90 decibels at close range and be heard clearly from three to five kilometres away through dense forest, which makes them the loudest land animals in the Americas and among the loudest anywhere. The volume comes from specialised anatomy. The hyoid bone, a small U-shaped bone in the throat that anchors the tongue, is hugely enlarged in howlers and hollowed into a rigid, cup-shaped shell that works as a resonating chamber, and the larynx and vocal folds are also greatly enlarged. Air forced past the vocal folds and through this chamber produces a deep, rolling roar or growl. Males have the largest hyoid and the deepest voices.
Why do howler monkeys howl?
Mainly to space troops out without fighting. Howlers live in groups that defend access to fruiting and leafing trees, but they are slow, leaf-fed animals that cannot afford the energy or injury risk of frequent physical battles. So each troop announces its presence with loud communal calls at dawn and dusk and whenever it hears or approaches another troop. A group that hears a rival roaring nearby can simply move away, and boundaries are settled by voice. The calls also help troop members keep track of each other in thick canopy, and a male's roar advertises his size and fitness.
Why are howler monkeys so inactive?
Their diet. Howlers eat a large proportion of leaves, which are abundant and easy to reach but low in usable energy and hard to digest, requiring slow bacterial fermentation in an enlarged hindgut. An animal running on that kind of fuel cannot sustain a busy, athletic lifestyle, so howlers have gone the other way — they rest and sleep for the great majority of the day, move slowly and economically when they do move, and let their gut do the work. This "energy minimiser" strategy lets them live in forests, and on parts of forests, that more active fruit-eating monkeys cannot exploit.
Are howler monkeys dangerous or aggressive?
Not to people. Despite the terrifying volume of their calls, howler monkeys are shy, slow, and non-confrontational, and they will move away from humans rather than approach. They do not throw things at people with any accuracy, do not raid camps aggressively, and have small canines compared with many monkeys. The roaring that unnerves people walking in the forest at dawn is directed at other howler troops, not at the listener. Where howlers have been habituated at lodges and reserves they generally ignore visitors entirely.